CLASS 10 SCIENCE NOTES: CHAPTER- LIGHT: REFLECTION AND REFRACTION

Topics covered: 

What is light? 

Laws of Reflection

Plane & Spherical Mirrors

Refraction

Lenses

What is Light?

• Light is a form of energy that enables us to see objects around us. Its speed depends on the medium through which it travels.
• The speed of light in vacuum is 3 × 10⁸ m/s.
• Light always travel in a straight line and this principle is known asRectilinear propagation.
• A bundle of light rays forms a beam of light.

Reflection of Light

The phenomenon of bouncing back of light from a polished surface (like a mirror) is called reflection.

Points to remember

• Incident ray: Incoming ray that strikes the surface.
• Reflected ray: Ray that bounces back after striking.
• Normal: Imaginary line perpendicular to the surface at the point of incidence.

Laws of Reflection

1. The angle of incidence (i) is equal to the angle of reflection (r): i = r.
2. The incident ray, normal and reflected ray all lie in the same plane.

Lateral Inversion

Lateral inversion is when the left and right sides of an object appear reversed in the image formed by a plane mirror.

For example, if you raise your right hand, the image appears to raise the left hand. The word AMBULANCE is written inverted on the front of ambulances so drivers ahead can read it in their rear-view mirror.

Characteristics of image formed by a plane mirror

• Image is virtual and erect.
• The size of image is same as that of object.
• The distance of image from mirror is same as the distance of object from mirror.
• Focal length of a plane mirror is infinite.

Spherical Mirrors

Mirrors whose reflecting surfaces are part of spheres are called spherical mirrors.

Key terms

Pole (P)	The exact center of the mirror's reflecting surface.
Centre of curvature (C)	The center of the sphere of which the mirror is a part.
Radius of curvature (R)	Radius of that sphere. R = 2f.
Principal axis	The straight line passing through P and C.
Principal focus (F)	The point on the principal axis where parallel rays meet (concave) or appear to diverge from (convex).
Focal length (f)	Distance between P and F. f = R / 2.
Aperture	How wide the reflecting surface is.

Ray rules (quick)

• Ray parallel to principal axis → after reflection passes through (or appears to) the focus.

• Ray through principal focus → becomes parallel to principal axis after reflection.

• Ray through centre of curvature → retraces its path after reflection.

• Ray incident at pole → reflected making equal angles with principal axis.

Image formation by concave mirror

Uses of concave mirror:

• Used by dentists: As the mirror forms enlarged and erect image when mirror is placed near the object.
• Used as solar furnaces: The concave mirrors focus sunlight at a single point, producing a large amount of heat.
• Used as headlight in vehicles: The concave mirrors are used in headlights, torches and searchlights to direct a parallel beam of light after placing the bulb at focus.

Image formation by convex mirror

Uses of convex mirror

• Rear-view mirrors in vehicles (wider field of view; always virtual and diminished).
• Security and surveillance mirrors in shops and parking areas.

Comparison: Concave vs Convex

Feature	Concave Mirror	Convex Mirror
Types of images	Real & inverted (mostly); virtual & erect when object is between P and F	Always virtual & erect
Image size	Diminished, same size, or enlarged (depends on position)	Always diminished
Nature of image	Real or virtual	Always virtual
Uses	Shaving/makeup mirrors, dentists, solar furnaces	Rear view mirrors, security mirrors

Sign Convention (Spherical Mirrors)

Sign convention helps when using mirror and magnification formulas to easily determine the position and nature of images formed by the spherical mirrors. Take the pole (P) as origin and principal axis as reference. 

• All distances measured from P.
• Distances measured in the direction of incident light (left → right) are positive.
• Distances opposite the direction of incident light (right → left) are negative.
• Heights above principal axis is positive. 
• Heights below principal axis is negative.

Summary

Quantity	Direction / Position	Sign Taken	Example / Note
Object distance (u)	Opposite to the direction of light (right to left)	Negative (-)	Object is usually placed in front of the mirror
Image distance (v)	In the direction of light (left to right)	Positive (+) if image is on same side as light; Negative (-) if on opposite side	Real image →negative; Virtual image →positive
Focal length (f)	In the direction of light (for convex mirror)	Positive (+)	Concave mirror → negative; Convex mirror → positive
Radius of curvature (R)	In the direction of light	Positive (+)	Concave → negative; Convex → positive
Height of image (h)	Above principal axis	Positive (+)	Virtual, erect image
Height of image (h)	Below principal axis	Negative (-)	Real, inverted image

Mirror formula

The mirror formula relates object distance (u), image distance (v) and focal length (f):

1/f = 1/v + 1/u

Magnification

Magnification (m) is given by:

m = h'/h = -v/u

where h' is image height and h is object height.

Refraction of Light

Refraction is the bending of light when it passes from one medium to another because its speed changes.

Key points

• Incident ray: ray in first medium.
• Refracted ray: ray in second medium after bending.
• Normal: perpendicular to surface at point of incidence.
• Light bends towards the normal when going from rarer → denser medium.
• Light bends away from the normal when going from denser → rarer medium.

Refraction through a rectangular glass slab

The path of light

• Entry Point (Air to Glass):The incident ray enters the glass from the air (rarer to denser). It bends TOWARDS the Normal, forming the angle of refraction (r).
• Exit Point (Glass to Air): The refracted ray reaches the opposite side and exits the glass back into the air (denser to rarer). It bends AWAY from the Normal. This final ray is called the emergent ray, and it forms the angle of emergence.

Lateral displacement: The perpendicular distance between the original path of an incident ray and its emergent path after passing through a transparent medium like a glass slab is called lateral displacement.

This sideways shift is caused by the change in the light ray's speed as it enters and exits the medium, and it depends on the thickness of the slab, its refractive index, and the angle of incidence.

Laws of Refraction (Snell's Law)

1. The incident ray, the refracted ray, and the normal all lie in the same plane.
2. The ratio of the sine of angle of incidence (i) to the sine of angle of refraction (r):                                  sin i / sin r = constant

This constant is the refractive index of the second medium with respect to the first.

Refractive index

The refractive index measures how much light bends when passing from one medium to another.

Spherical Lenses

Spherical lenses are formed by joining two spherical surfaces. There are two types of lenses: 

(a) convex lens (converging) and

(b) concave lens (diverging).

Term	Simple Definition
Principal Axis	Imaginary line passing through the center of the lens.
Aperture	Diameter (width) of the lens' circular edge through which refraction occurs.
Optical Center (O)	Center point of the lens; light passes undeviated, light goes straight without bending.
Center of Curvature (C1 and C2)	Centers of the spheres from which the lens surfaces are made.
Radii of Curvature (R1 and R2)	The distance from the lens's center to each C1 or C2. A lens has two (R1 and R2). This is equal to twice the focal length. (R= 2f)
Principal Focus - Convex Lens (F1, F2)	Point where parallel rays meet (or converge) after passing through the lens. A convex lens has two foci.
Principal Focus - Concave Lens (F1, F2)	Point from which parallel rays appear to spread out (or diverge); located behind the lens. A concave lens has two foci.

Ray Tracing Rules for Lenses

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Rule 1: Parallel Ray

• Convex Lens: A ray parallel to the principal axis passes through the other-side principal focus (F₂) after refraction.
• Concave Lens: A ray parallel to the principal axis appears to diverge from the near-side principal focus (F₁) after refraction.

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Rule 2: Ray through Focus

• Convex Lens: A ray passing through the near-side principal focus (F₁) emerges parallel to the principal axis after refraction.
• Concave Lens: A ray directed toward (appearing to meet at) the  other-side principal focus (F₂) emerges parallel to the principal axis after refraction.

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Rule 3: Ray through Optical Center

• For all lenses (Convex and Concave): A ray of light passing through the optical center(O) of the lens goes undeviated (it passes straight through without bending).

IMAGE FORMATION BY CONVEX LENS

S.N.	Position of the object	Position of the image	Relative size of the image	Nature of the image
a)	At infinity	At focus F2	Highly diminished, point-sized	Real and inverted
b)	Beyond 2F1	Between F2 and 2F2	Diminished	Real and inverted
c)	At 2F1	At 2F2	Same size	Real and inverted
d)	Between F1 and 2F1	Beyond 2F2	Enlarged	Real and inverted
e)	At focus F1	At infinity	Image would not be formed	Real and inverted
f)	Between focus F1 and optical centre O	On the same side of the lens as the object	Enlarged	Virtual and erect

Uses of Convex Lens

A Convex Lens (thicker in the middle) is mainly used to magnify things or focus light.

• Magnifying Glass: 
  It makes small objects look enlarged, virtual, and erect when you hold the object close to the lens.

• Correcting Hypermetropia (Far-Sightedness): 
  Used in eyeglasses to help people who cannot see nearby objects clearly by increasing the eye's focusing power.

• Microscopes and Telescopes: 
  It is a key part of these tools, used in both the objective and eyepiece systems to magnify small or distant objects.

IMAGE FORMATION BY CONCAVE LENS

S.N.	Position of the object	Position of the image	Relative size of the image	Nature of the image
a)	At infinity	At focus F1	Highly diminished, point-sized	Virtual and erect
b)	Between infinity and optical centre O of the lens	Between focus F1 and optical centre O	Diminished	Virtual and erect

Uses of Concave Lens

A Concave Lens (thinner in the middle) is used to spread light (diverge rays) for different applications:

• Correcting Myopia (Near-Sightedness):
  Used in spectacles to help people who cannot see distant objects clearly by spreading the light before it reaches the eye.

• In Optical Instruments:
  Used in devices like peep-holes, binoculars, and telescopes to diverge light rays, helping control the image size and clarity.

• Laser Flashlights and Beam Expanders:
  Used in many flashlights and laser systems where a diverging or expanding beam of light is required.

Lens Sign Convention Summary

The New Cartesian Sign Convention is used to assign positive or negative signs to distances and heights in lenses. It ensures correct use of the Lens Formula and Magnification Formula.

Rule	Description	Sign
Origin	All distances are measured from the Optical Centre (O) of the lens.	N/A
Incident Light Direction	Distances measured in the same direction as the incident light are taken as Positive.	→ Positive
Opposite Light Direction	Distances measured in the direction opposite to the incident light are taken as Negative.	← Negative
Upward Height	Heights measured upward and perpendicular to the principal axis are taken as Positive.	↑ Positive
Downward Height	Heights measured downward and perpendicular to the principal axis are taken as Negative.	↓ Negative

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Focal Lengths of Lenses

• Convex Lens (Converging): Focal length (f) is Positive.
  Reason: Parallel rays of light converge at the principal focus (F₂) on the right side of the lens, in the direction of incident light.


• Concave Lens (Diverging): Focal length (f) is Negative.
  Reason: Parallel rays of light appear to diverge from the principal focus (F₁) on the left side of the lens, opposite to the direction of incident light.

 Lens Formula

The relationship between the object distance (u), image distance (v), and focal length (f) is given by:

1/f = 1/v - 1/u

• All distances are measured from the optical centre (O).
• Sign convention is used to determine the correct signs of u, v, and f.
• Convex Lens: f is positive.
• Concave Lens: f is negative.

Magnification Formula

Magnification (m) represents how much larger or smaller the image is compared to the object.

m = h_i / h_o = v / u

• h_i = Height of the image
• h_o = Height of the object
• m > 1 → Image is enlarged
• m < 1 → Image is diminished
• Negative m → Image is real and inverted
• Positive m → Image is virtual and erect

POWER OF LENS

The Power of a lens (P) indicates its ability to converge or diverge light.

It is mathematically defined as the reciprocal of its focal length (f).

Formulas:

• P = 1 / f (where f is in metres (m))
• P = 100 / f (where f is in centimetres (cm))

S.I. Unit:

The S.I. unit of power is the Diopter (D).

1 Diopter (1 D) is the power of a lens whose focal length is 1 metre (1 m).

Signs for Power (P)

• Power for Convex Lens: Positive (+), because the focal length (f) for a convex lens is also positive.
• Power for Concave Lens: Negative (-), because the focal length (f) for a concave lens is also negative.

COMBINATION OF LENSES

When two or more thin lenses are placed in contact with each other, their combined power (P_{total}) is the algebraic sum of the powers of the individual lenses.

P_{total} = P_1 + P_2 + P_3 + ...

(Where P_1, P_2, P_3, .... are the powers of the individual lenses.)