YRS : Light - Notes

🌟 LIGHT — COMPLETE DETAILED NOTES

Class 8–10 | CBSE / DAV / Board Exam Preparation

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1. What is Light?

Light is a form of energy that enables us to see objects.

We see an object when light coming from it enters our eyes.

Sources of Light

Luminous objects: Objects that produce their own light.

Examples:

• Sun
• Stars
• Candle flame
• Electric bulb
• Firefly

Non-luminous objects: Objects that do not produce their own light but can be seen because they reflect light.

Examples:

• Moon
• Book
• Chair
• Tree
• Wall

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2. Natural and Artificial Sources of Light

Natural Sources

Sources occurring naturally in nature.

Examples:

• Sun
• Stars
• Lightning
• Fireflies

Artificial Sources

Sources made or controlled by humans.

Examples:

• Electric bulb
• LED
• Torch
• Candle
• Tube light

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3. Rectilinear Propagation of Light

Light travels in a straight line through a homogeneous medium.

This property is called rectilinear propagation of light.

Evidence

• Formation of shadows
• Pinhole camera
• Solar eclipse
• Lunar eclipse

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4. Beam of Light

A group of light rays travelling in a particular direction is called a beam of light.

Types of Beams

Parallel beam

Rays travel parallel to each other.

Convergent beam

Rays move towards a common point.

Divergent beam

Rays move away from a common point.

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5. Ray of Light

A ray of light is the straight-line path followed by light in a transparent homogeneous medium.

It is represented by an arrow:

────────────→

The arrow shows the direction of propagation.

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6. Materials According to Transmission of Light

Transparent

Allows most light to pass through.

Examples:

• Clear glass
• Clean water
• Air

Objects can usually be seen clearly through transparent materials.

Translucent

Allows only some light to pass through.

Examples:

• Frosted glass
• Butter paper
• Thin curtain

Objects cannot be seen clearly.

Opaque

Does not allow light to pass through.

Examples:

• Wood
• Brick
• Metal
• Stone

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7. Shadow

A shadow is a dark region formed when an opaque object blocks the path of light.

Conditions for formation

Three things are required:

1. Light source
2. Opaque object
3. Screen

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8. Umbra and Penumbra

Umbra

The completely dark central region of a shadow.

Penumbra

The partially illuminated region surrounding the umbra.

These are especially important in understanding eclipses.

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9. Eclipse

An eclipse occurs when one celestial body comes between a light source and another body, causing a shadow.

Solar Eclipse

The Moon comes between the Sun and Earth.

Sun → Moon → Earth

Lunar Eclipse

The Earth comes between the Sun and Moon.

Sun → Earth → Moon

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10. Reflection of Light

When light strikes a surface and returns into the same medium, the phenomenon is called reflection.

Examples

• Mirror
• Calm water
• Polished metal

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11. Laws of Reflection

First Law

$$\boxed{{\displaystyle \mathrm{\angle }i=\mathrm{\angle }r}}$$

The angle of incidence is equal to the angle of reflection.

Second Law

The incident ray, reflected ray and normal at the point of incidence lie in the same plane.

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12. Important Reflection Terms

Incident Ray

Ray falling on the reflecting surface.

Reflected Ray

Ray returning after reflection.

Normal

A perpendicular line drawn to the reflecting surface at the point of incidence.

Angle of Incidence

Angle between incident ray and normal.

Angle of Reflection

Angle between reflected ray and normal.

Important: Angles are measured from the normal, not from the mirror surface.

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13. Regular and Diffuse Reflection

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Regular Reflection

Occurs from a smooth surface.

Reflected rays remain orderly/parallel for parallel incident rays.

Example: Plane mirror.

Diffuse Reflection

Occurs from a rough surface.

Reflected rays scatter in different directions.

Example: Wall.

Important: Diffuse reflection still obeys the laws of reflection at each point on the surface.

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14. Plane Mirror

A plane mirror has a flat reflecting surface.

Image Characteristics

The image formed by a plane mirror is:

• Virtual
• Erect
• Same size as object
• Laterally inverted
• At the same distance behind the mirror as the object is in front

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15. Lateral Inversion

The apparent left-right reversal of an object in a plane mirror is called lateral inversion.

Example:

If you raise your right hand, your mirror image appears to raise its left hand.

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16. Multiple Reflection

When light undergoes reflection repeatedly from more than one reflecting surface, it is called multiple reflection.

Applications

• Periscope
• Kaleidoscope
• Some optical instruments

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17. Number of Images Between Two Plane Mirrors

When two plane mirrors are inclined at an angle $\theta$, the approximate number of images can be determined using:

$$\boxed{{\displaystyle N=\frac{{360}^{\circ }}{\theta }-1}}$$

when ${360}^{\circ }\mathrm{/}\theta$ is an integer.

For example, if:

$$\theta ={90}^{\circ }$$

then:

$$N=\frac{360}{90}-1=3$$

So, 3 images are formed.

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18. Spherical Mirrors

A spherical mirror is a portion of a spherical reflecting surface.

Two types:

Concave Mirror

Reflecting surface curves inward.

Convex Mirror

Reflecting surface bulges outward.

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19. Parts of a Spherical Mirror

Pole — P

Centre of the reflecting surface.

Centre of Curvature — C

Centre of the sphere of which the mirror is a part.

Radius of Curvature — R

Distance between P and C.

$$R=PC$$

Principal Axis

Straight line passing through P and C.

Principal Focus — F

Point where rays parallel to the principal axis converge or appear to diverge from after reflection.

Focal Length — f

Distance between P and F.

$$f=PF$$

For a spherical mirror:

$$\boxed{{\displaystyle R=2f}}$$

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20. Ray Rules for Concave Mirror

Rule 1

A ray parallel to the principal axis passes through the focus after reflection.

Rule 2

A ray passing through the focus becomes parallel to the principal axis after reflection.

Rule 3

A ray passing through C retraces its path.

Rule 4

A ray striking the pole follows the law of reflection.

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21. Image Formation by Concave Mirror

Object Position	Image Position	Nature
Beyond C	Between C and F	Real, inverted, diminished
At C	At C	Real, inverted, same size
Between C and F	Beyond C	Real, inverted, enlarged
At F	At infinity	Highly enlarged
Between F and P	Behind mirror	Virtual, erect, enlarged

This table is very important for examinations.

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22. Convex Mirror

A convex mirror always forms a:

• Virtual image
• Erect image
• Diminished image

The image appears behind the mirror between P and F.

Main Advantage

It provides a wide field of view.

Therefore it is commonly used as a rear-view mirror in vehicles.

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23. Mirror Formula

For spherical mirrors:

$$\boxed{{\displaystyle \frac{1}{f}=\frac{1}{v}+\frac{1}{u}}}$$

Where:

• $u$ = object distance
• $v$ = image distance
• $f$ = focal length

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24. Magnification of Mirror

$$\boxed{{\displaystyle m=\frac{h_i}{h_o}=-\frac{v}{u}}}$$

Where:

• $h_i$ = image height
• $h_o$ = object height

Interpretation

If $m>1$: enlarged image.

If $m<1$: diminished image.

If $m$ is positive: image is erect.

If $m$ is negative: image is inverted.

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25. Refraction of Light

The bending of light when it passes obliquely from one transparent medium to another is called refraction.

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26. Refraction Rules

Rarer → Denser

Light bends towards the normal.

Example:

Air → Glass

Denser → Rarer

Light bends away from the normal.

Example:

Glass → Air

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27. Why Does Refraction Occur?

Refraction occurs because the speed of light changes when it enters a different optical medium.

The frequency of light remains unchanged when it crosses the boundary, while its speed and wavelength change.

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28. Refractive Index

$$\boxed{{\displaystyle n=\frac{c}{v}}}$$

Where:

• $n$ = refractive index
• $c$ = speed of light in vacuum
• $v$ = speed of light in the medium

A larger refractive index generally indicates that light travels more slowly in that medium.

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29. Snell's Law

For two given media:

$$\boxed{{\displaystyle \frac{\sin i}{\sin r}=n_{21}}}$$

where $n_{21}$ is the refractive index of medium 2 relative to medium 1.

A commonly used form is:

$$\boxed{{\displaystyle n_1\sin i=n_2\sin r}}$$

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30. Refraction Through a Glass Slab

When light passes through a rectangular glass slab:

• It bends towards the normal on entering glass.
• It bends away from the normal on leaving glass.
• The emergent ray is parallel to the incident ray.
• There is a sideways shift called lateral displacement.

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31. Total Internal Reflection

When light travelling from a denser medium to a rarer medium strikes the boundary at an angle greater than the critical angle, it is completely reflected back into the denser medium.

This is called total internal reflection (TIR).

Conditions

1. Light must travel from denser to rarer medium.
2. Angle of incidence must be greater than the critical angle.

Applications

• Optical fibres
• Medical endoscopes
• Telecommunications
• Some prisms in optical instruments

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32. Dispersion

Splitting of white light into its constituent colours is called dispersion.

$$\boxed{{\displaystyle \text{White Light}\to VIBGYOR}}$$

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33. Order of Colours

$$\boxed{{\displaystyle V-I-B-G-Y-O-R}}$$

Deviation

Violet: maximum deviation

Red: minimum deviation

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34. Rainbow Formation

A rainbow involves:

1. Refraction
2. Dispersion
3. Internal reflection
4. Refraction again as light exits the raindrop

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35. Scattering of Light

Scattering is the redirection of light in different directions by particles in a medium.

Examples

• Blue sky
• Red/orange sunrise and sunset
• Visibility of light beams in fog

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36. Why is the Sky Blue?

Sunlight contains many colours.

The atmosphere scatters shorter wavelengths more strongly than longer wavelengths.

Blue light is scattered strongly, so the sky appears predominantly blue during the daytime under ordinary atmospheric conditions.

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37. Why Does the Sun Look Red at Sunrise and Sunset?

At sunrise and sunset, sunlight travels through a longer path in the atmosphere.

More shorter-wavelength light is scattered away from the direct path.

More red/orange light reaches the observer.

Therefore the Sun often appears reddish or orange.

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38. Tyndall Effect

The scattering of light by colloidal or very fine particles, making the path of a beam visible, is called the Tyndall effect.

Examples:

• Beam of sunlight in a dusty room
• Car headlights in fog
• Projector beam in a smoky room

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39. Human Eye

The human eye is a natural optical instrument.

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Main Parts

Cornea: Transparent front surface; provides much of the eye's focusing power.

Iris: Controls the size of the pupil.

Pupil: Opening through which light enters.

Lens: Focuses light on the retina.

Ciliary muscles: Adjust the shape of the lens.

Retina: Light-sensitive layer where the image is formed.

Optic nerve: Carries visual information to the brain.

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40. Image Formation in Human Eye

The eye lens forms a real, inverted and diminished image on the retina.

The brain processes the signals and gives us the perception of an upright world.

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41. Accommodation

The ability of the eye lens to change its focal length so that objects at different distances can be focused on the retina is called accommodation.

Normal Eye

For a normal eye:

Near point = 25 cm

Far point = infinity

The near point is called the least distance of distinct vision for a typical normal adult eye.

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42. Myopia

Myopia is also called short-sightedness.

A person can see nearby objects clearly but distant objects appear blurred.

Correction

Use a:

$$\boxed{{\displaystyle \text{Concave lens}}}$$

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43. Hypermetropia

Hypermetropia is also called long-sightedness.

A person can see distant objects clearly but nearby objects appear blurred.

Correction

Use a:

$$\boxed{{\displaystyle \text{Convex lens}}}$$

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44. Presbyopia

Presbyopia is an age-related loss of accommodation.

It can make nearby objects difficult to see clearly.

Correction may involve:

• Convex lenses
• Bifocal lenses
• Progressive lenses

depending on the individual's visual needs.

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45. Cataract

A cataract is clouding of the eye's natural lens.

It can cause:

• Blurred vision
• Reduced clarity
• Glare sensitivity

Treatment commonly involves surgical removal of the cloudy lens and replacement with an artificial intraocular lens.

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46. Lenses

A lens is a transparent optical medium bounded by two surfaces, at least one of which is curved.

Convex Lens

Thicker in the middle and thinner at the edges.

It is a converging lens.

Concave Lens

Thinner in the middle and thicker at the edges.

It is a diverging lens.

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47. Ray Rules for Convex Lens

Rule 1

A ray parallel to the principal axis passes through the principal focus after refraction.

Rule 2

A ray passing through the optical centre travels approximately undeviated.

Rule 3

A ray passing through the principal focus emerges parallel to the principal axis.

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48. Image Formation by Convex Lens

Object Position	Image Position	Nature
Beyond 2F	Between F and 2F	Real, inverted, diminished
At 2F	At 2F	Real, inverted, same size
Between F and 2F	Beyond 2F	Real, inverted, enlarged
At F	At infinity	Highly enlarged
Between O and F	Same side of lens	Virtual, erect, enlarged

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49. Concave Lens

A concave lens always forms a:

• Virtual image
• Erect image
• Diminished image

for a real object.

It is commonly used to correct myopia.

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50. Power of Lens

Power measures the converging or diverging ability of a lens.

$$\boxed{{\displaystyle P=\frac{1}{f}}}$$

Here $f$ is in metres.

Unit

$$\boxed{{\displaystyle \text{Dioptre (D)}}}$$

Sign

Convex lens:

$$P>0$$

Concave lens:

$$P<0$$

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51. Important Numerical

A convex lens has focal length:

$$f=20cm$$

Convert:

$$20cm=0.20m$$

Therefore:

$$P=\frac{1}{0.20}$$$$\boxed{{\displaystyle P=+5D}}$$

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52. Combination of Lenses

For thin lenses placed together:

$$\boxed{{\displaystyle P=P_1+P_2+P_3+\cdots }}$$

Example:

$$P_1=+2D,P_2=+3D$$

Then:

$$P=2+3$$$$\boxed{{\displaystyle P=+5D}}$$

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53. Important Formula Sheet

Reflection

$$\boxed{{\displaystyle i=r}}$$

Mirror

$$\boxed{{\displaystyle R=2f}}$$

Mirror Formula

$$\boxed{{\displaystyle \frac{1}{f}=\frac{1}{v}+\frac{1}{u}}}$$

Mirror Magnification

$$\boxed{{\displaystyle m=-\frac{v}{u}}}$$

Lens Formula

$$\boxed{{\displaystyle \frac{1}{f}=\frac{1}{v}-\frac{1}{u}}}$$

Lens Magnification

$$\boxed{{\displaystyle m=\frac{v}{u}}}$$

Lens Power

$$\boxed{{\displaystyle P=\frac{1}{f}}}$$

Refractive Index

$$\boxed{{\displaystyle n=\frac{c}{v}}}$$

Snell's Law

$$\boxed{{\displaystyle n_1\sin i=n_2\sin r}}$$

Lens Combination

$$\boxed{{\displaystyle P=P_1+P_2+\cdots }}$$

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⭐ 54. Super Important Exam Points

1. Light travels in a straight line in a homogeneous medium.
2. Reflection means bouncing back of light.
3. $i=r$.
4. Angles of incidence and reflection are measured from the normal.
5. Plane mirror forms a virtual, erect and same-size image.
6. Concave mirror can form real or virtual images depending on object position.
7. Convex mirror gives a wide field of view.
8. Refraction occurs because the speed of light changes between media.
9. Denser medium → light generally bends towards normal.
10. Rarer medium → light generally bends away from normal.
11. White light consists of a continuous range of visible wavelengths; VIBGYOR is a common school representation of its principal colour bands.
12. Violet deviates more than red in a prism.
13. A convex lens converges light.
14. A concave lens diverges light.
15. Myopia → concave lens.
16. Hypermetropia → convex lens.
17. Power of convex lens is positive.
18. Power of concave lens is negative.
19. Normal eye's near point is approximately 25 cm.
20. Retina is the light-sensitive layer.
21. Optical fibre works on total internal reflection.
22. Blue sky is associated with stronger scattering of shorter wavelengths.
23. Red/orange sunsets result from wavelength-dependent scattering over a longer atmospheric path.
24. A rainbow involves refraction, dispersion and internal reflection.
25. A real image can generally be obtained on a screen; a virtual image cannot.

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📝 20 Practice Questions

1. Define light.
2. What is rectilinear propagation?
3. Define reflection.
4. State the two laws of reflection.
5. Differentiate regular and diffuse reflection.
6. What is lateral inversion?
7. Write four characteristics of a plane-mirror image.
8. Define principal focus and focal length.
9. Give four uses of concave mirrors.
10. Why are convex mirrors used in vehicles?
11. Define refraction.
12. What happens when light travels from air to glass?
13. State Snell's law.
14. What is dispersion?
15. Name the colours of the visible spectrum in order.
16. Explain why the sky appears blue.
17. What is accommodation of the eye?
18. What is myopia and how is it corrected?
19. What is hypermetropia and how is it corrected?
20. Calculate the power of a lens whose focal length is $25cm$.

Answer to Q20

$$f=25cm=0.25m$$$$\mathrm{<br>}\mathrm{<br>}P=\frac{1}{f}$$$$\mathrm{<br>}\mathrm{<br>}P=\frac{1}{0.25}$$$$\boxed{{\displaystyle P=+4D}}$$

This is the full chapter-style Light notes covering reflection, mirrors, refraction, dispersion, scattering, human eye, defects of vision, lenses, formulas, ray diagrams and numericals.