CLASS 10 NOTES

CLASS 10 NOTES

PRASHANT KIRAD

SCIENCE

Light

REFLECTION

The phenomenon where light rays return to the same medium upon

striking a surface is known as "reflection."

Laws of Reflection:

The First Law of Reflection: The incident ray, the reflected ray, and the

normal to the surface at the point of incidence, all lie in the same plane.

The Second Law of Reflection: The angle of incidence is equal to the angle of reflection. In other words, the angle between the incident ray and the normal is equal to the angle between the reflected ray and the normal.

*These laws of reflection are applicable to the reflection of light from any

smooth surface, not just mirrors.*

Properties of Image Formed by a Plane Mirror:

1. An image formed by a plane mirror is virtual and erect.

2. The Image is laterally inverted

3. The image formed is as far behind the mirror as

the object is in front of it

4. The Size of the image is Equal to that of the Object

(m=1)

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Spherical Mirrors:

Mirrors, Whose reflecting surfaces are spherical in the part of a hollow

sphere of glass. A Concave mirror is a curved mirror where the reflecting

surface is on the inner side of the curved shape. Convex Mirror is a curved

mirror where the reflective surface bulges out toward the light source.

Sp�e��c�� Mir���s:

(1) Concave mirror - Whose Reflecting Surface is curved inwards is called a concave mirror.

(2) Convex mirror -Whose Reflecting Surface is curved outwards is called a convex mirror.

Terms used in Spherical mirrors:

●

● Pole: The center of a spherical mirror's reflecting surface coincides with the mirror's surface itself. Typically, we denote the mirror's pole with the letter "P."

● Centre of curvature: A spherical mirror has a curved surface from a sphere. The center of that sphere is the center of curvature. In concave mirrors, it's in front, in convex mirrors, it's behind.

● Radius of curvature: The sphere's radius, from which the reflecting

surface of a spherical mirror is derived, is symbolized by the letter "R."

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● Principal axis: The principal axis of a spherical mirror is a straight line

that extends through the mirror's pole and its center of curvature. This

axis is perpendicular to the mirror's surface at its pole

● Principal Focus: Parallel rays meet at the principal focus (F) for a concave mirror and seem to diverge from the principal focus (F) for a convex mirror.

The distance between the mirror's center and the principal focus is the focal length.

● Aperture: The reflecting surface's diameter in a spherical mirror is called the aperture. When the aperture is much smaller than the radius of curvature, we can use R = 2f as an approximation.

Ray Diagrams Rules:

(1)A parallel ray will either pass through or appear to converge at the

principal focus for a concave mirror and appear to diverge from the

principal focus for a convex mirror.

2. A ray directed at the principal focus of a concave mirror or passing

through the principal focus of a convex mirror will come out parallel to

the principal axis.

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3. A ray passing through the center of curvature of a concave mirror or

directed toward the center of curvature of a convex mirror will be

reflected back along its original path

4. Ray incident obliquely to the principal axis towards the pole of the

concave mirror on a convex mirror is Reflected Obliquely.

IMAGE FORMATION

BY CONVEX MIRROR:

2)

(1)

(2)

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IMAGE FORMATION

BY CONCAVE MIRROR:

Uses of Spherical Mirrors: Con����: f= (-ve)

Concave mirrors are used in torches, headlights, shaving mirrors, dental examinations, and solar furnaces for heat.

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Con���: f=(+ve)

Convex mirrors are used in vehicles’s rear-view mirrors to provide smaller

but upright images and a wider field of view, enhancing driver visibility.

Sign-convention:

1. Object on the left, light comes from the left.

2. Start measuring from the mirror's pole.

3. Left is negative, right is positive.

4. Above the principal axis is positive.

5. Below the principal axis is negative.

Important:

● The object distance u, is always negative.

● The image distance v, is positive if the image is formed

behind a concave mirror and negative if the image is

formed in front of the mirror.

● The image distance v, is always positive for a convex

mirror.

● The focal length of a concave mirror is always negative

and that of a convex mirror is always positive.

● The height of an object is always positive.

● If the image is erect the height is taken as positive and if

the image is inverted, the height is taken as negative.

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MIRROR FORMULA:

● The distance of the Object from its pole is called the Object distance (u) ● The distance of the Image from its pole is called the image distance (v) ● The distance of the principal focus from the pole is called the focal length (f)

Magnification (m):

Magnification Produced by a spherical mirror gives the relative Extent to

which the image of an Object is magnified with respect to the Object's

size.

1. 0<m<1: Diminished ( Between 0 and 1)

2. m=1: Same Size

3. m>1: Enlarged

#An object is placed at a distance of 8cm from a convex mirror of a

focal length of 12cm. Find the position of the image.

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Ans: Also, in the case of a concave mirror, the magnification will be negative

because the object and mage will be above and below the principal axis.

So, m=-4 = Hi /Ho (Given)

Since the object is in front of a mirror (opposite to the direction of the ray of

light), it will be negative U = -12 cm (Given)

Find the distance of the image (v).

m= -4 = v/-12

Therefore V=-48 (Ans)

*A negative sign indicates that the image forms in front of the mirror.

REFRACTION

A Change in the path of a light ray as it passes from one medium to

another medium is called the Refraction of light.

LAWS OF REFRACTION:

● The incident ray, the reflected ray, and the normal to the

surface of separation of two media at the same point of

incidence, all lie in the same plane.

● Snell’s law: The ratio of sine of angle of incidence to the sine of

angle of refraction for a light of a given color is constant for a

given pair of media.

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Refraction through a Rectangular Glass slab:

i= Incident Ray

r= Reflected Ray

e= Emergent Ray

● Angle of incidence = Angle of emergence (∠i)=(∠e)

● When a ray of light is incident perpendicularly on

a plane glass slab, it passes through undeviated. In

this case, the angle of incidence (∠i) is 0°, and

therefore, the angle of refraction (∠r) is also 0°.

Refractive Index:

The refractive index measures how light changes direction when it

moves from air to another material. It shows how fast or slow light

travels and bends in different substances, helping us understand its

behavior.

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Absolute refractive index:

The absolute refractive index is the refractive index of a substance

compared to a vacuum (where the first medium is free space or a vacuum).

The refractive index of water is 1.33, which means that light travels about

1.33 times slower in water compared to its speed in air.

Spe���� Qu�s��o�: #

Sol���o�:

#

Sol���o�:

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Spherical Lens:

A spherical lens is an optical lens with a curved surface that causes light rays to converge or diverge.

1 . Thick at middle

1. Thin at middle

2. Converging Lens

2. Diverging lens

(1) Concave lens - A concave lens is thin in the middle and thicker at the edges, and it makes light spread out. (2) Convex lens - A convex lens has a thicker middle and thinner edges, and it makes light converge.

Terms used in Spherical Lens:

● Center of curvature (c): The center of curvature for a spherical lens is the point on the principal axis that is at the same distance from the lens as the radius of curvature.

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● Principal Axis: An imaginary straight line passing through the two centers of curvature of a lens is called its principal Axis.

● Principal focus: The point where parallel rays meet (convex lens) or appear to diverge from (concave lens). Lenses have two such points.

● Aperture: The aperture of a spherical lens is its effective diameter, representing the size of the circular outline.

● Optical center (O): The optical center of a lens is where light passes through without bending.

● Focal length: Focal length is the distance between the principal focus and the optical center.

RAY DIAGRAMS:

Rul��:

● Rays parallel to the principal axis converge at the principal focus after refraction in a convex lens, while they appear to diverge from the principal focus in a concave lens.

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● A ray passing through or directed to the focus will emerge parallel to the principal axis.

● A ray directed towards the optical center will emerge without deviation.

Image formation by Convex Lens:

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Image formation by Concave Lens:

Lens formula & Magnification:

● u= image distance ● v = object distance, ● f= focal length

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Note: We apply sign conventions similar to those used for spherical mirrors, with the exception that all measurements are taken from the optical center of the lens.

Magnification - Ratio of the height of the Image and the height of the Object Represented by (m)

hi - height of the image ho- height of the object

(-Ve) Sign shows that the image is virtual & erect. (+Ve) Sign shows that the image is real & inverted.

Power of Lens:

The power of a lens is a measure of its ability to converge or diverge light and is defined as the reciprocal of its focal length (F) in meters. It is typically measured in diopters (D) and is calculated using the formula:

f - focal length in meter.

SI unit => (D) Dioptre

PRASHANT KIRAD

# TOP 7 QUESTIONS

Q.1) As the velocity of light increases, the refractive index of the medium decreases.

Light enters from air to water having a refractive index of 4/3. Find the speed of light in water. The speed of light in a vacuum is 3*10^{8} m/s. [CBSE 2012] (2 - Marks)

Sol���o�:

Q.2)An object is placed at the focus of a convex lens. Draw a ray diagram to locate the

position of the image formed, if any. State its position and nature. [CBSE 2013] (2-Marks)

Sol���o�:

Q.3) What is meant by the power of a lens? Give its SI unit. When two or more lenses are

placed in contact, what will their combined power? [CBSE 2012] (2-Marks)

Sol���o�:

Q.4) Differentiate between reflection and refraction of light.

Sol���o�:

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Q.5) (a) Name the spherical mirror used as: [CBSE 2012] (2-Marks)

1. Shavingmirror,

2. Rearview mirror in vehicles,

3. Reflector in search – fights.

(b) Write any three differences between a real and virtual image

Sol���o�: (a)

(b)

Q.6) A 2.0 cm tall object is placed perpendicular to the principal axis of a convex lens of

1 focal length 10 cm. The distance of the object from the lens is 15 cm. Find the

position, nature, and size of the image forms. [All India 2013] (5-Marks) Sol���o�:

PRASHANT KIRAD

Q.7) (a) A concave mirror produces a three-times enlarged image of an object placed 10

cm in front of it Calculate the focal length of the mirror.

(b) Show the formation of the image with the help of a ray diagram when the object is

placed 6 cm away from the pole of a convex mirror.

Sol���o�:

# Competency-based Question-Answer:

Q.1) A rear-view mirror is a device that allows the driver to see the traffic on the road

behind him. It usually finds its place at the top of the

windscreen inside the cabin. This device is one of the

most basic but essential safety devices in the vehicle.

It provides assistance to the driver during overtaking,

parking in reverse gear, etc. Generally, vehicles also

have a pair of mirrors attached to the body from the

outside. They are known as 'side mirrors or Outer

Rear View Mirrors (ORVM) which serve the same

purpose. Almost all modern cars mount their side mirrors on the doors-normally at A-pillar

rather than the wings (the portion of the body above the wheel well).

PRASHANT KIRAD

(a) What type of Mirror is required to make ORVM in the vehicles?

(b) What type of image is formed by such a mirror?

(C) Draw a ray diagram to show the formation of an image by this type of mirror.

(d) Why are these types of mirrors used as rear-view mirrors in vehicles?

Sol���o�:

(a) Convex Mirror

(b) Convex mirror always forms an erect, virtual, and diminished image for all positions of

the object placed in front of it.

(c)

(d) Convex mirrors are used as rear-view mirrors in vehicles to see the traffic at the rear

side (or back side) because- (i) a convex mirror always produces an erect image of the

object; (ii) the image formed in a convex mirror is highly diminished due to which a convex

mirror gives a wide field of view.

Q.2) The above images are those of a specialized slide projector. Slides are small

transparencies mounted in

sturdy frames ideally suited

to magnification and

projection since they have a

very high resolution and a

high image quality. There is

a tray where the slides are

to be put into a particular

orientation so that the

viewers can see the enlarged erect images of the transparent slides. This means that the

slides will have to be inserted upside down in the projector tray. To show her students

the images of insects that she investigated in the lab, Mrs. Iyer brought a slide

projector. Her slide projector produced a 500-times enlarged and inverted image of a

slide on a screen 10 m away. (CBSE 22-23 SQPs)

(a) Based on the text and data given in the above paragraph, what kind of lens must the

slide projector have?

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(b) If v is the symbol used for image distance and u for object distance then with one

reason state what will be the sign for 𝑣/u in the given case.

(c) A slide projector has a convex lens with a focal length of 20 cm. The slide is placed

upside down 21 cm from the lens. How far away should the screen be placed from the

slide projector’s lens so that the slide is in focus?

(d) When a slide is placed 15 cm behind the lens in the projector, an image is formed 3 m

in front of the lens. If the focal length of the lens is 14 cm, draw a ray diagram to show

image formation. (not to scale)

(d) When a slide is placed 15 cm behind the lens in the projector, an image is formed 3 m

in front of the lens. If the focal length of the lens is 14 cm, draw a ray diagram to show

image formation. (not to scale)

Sol���o�:

(a) The slide projector has a Convex lens.

(b) Sign of v/u i.e. "m" will be negative as the image formed is real and

inverted.

c) Given f=20cm, u = -21 cm; v (screen distance) = ?

using lens formula 1/u + 1/f = 1/v

=> 1/v = 1/20 + 1/-21

=> 1/v = 21-20/420 = 1/420 v = 420 cm or 4.2 m

So, the screen be placed 4.2 m away from the lens of the projector.

(d)

Here the slide is placed between F and 2F of the lens and the image is

formed beyond 2F' of lens on its other side.

PRASHANT KIRAD