# Light Reflection and Refraction

## Introduction to Class 10 Science Notes Chapter 1:

Hello students, In this article I am sharing with you Class 10 Science Notes Chapter 1

## Class 10 Science Notes

Q 1.Define light: It is a form of energy which produces sensation of sight in us.

Q 2. Reflection of light
When a Ray of light comes from any source and falls on any reflecting surface after striking it goes back towards the same medium is called reflection of light.

Q 3. Laws of reflection:

First law: It states that the angle of incidence is always equal to angle of reflection i,e

< I = < r

Second law: It states that the angle of incidence, angle of reflection and the normal all lie on the same.

Q 4.Concave mirror: It is a part of hollow sphere whose reflecting surface is covered inwards and outward surface is polished.

Convex mirror: it is a part of hollow sphere whose reflecting surface is covered outwards and inner surface is polished.

Q 5. Uses of concave mirrors and convex mirrors.

Uses of concave mirrors:
*It can be used in torches.
*It can be used in solar furnaces.
*It can be used in shaving minors.
*It can be also used by dentist.
*It can be used in vehicles as headlight.

Uses of convex mirror:

*It can be used in vehicles as rear view mirror that we can see larger area clearly and also for by objects near to us.
*It can be used by detectives.

Q 6. Refraction of light: The bending of light rays when they pass obliquely from one medium (rearer) to another medium (denser) is called refraction of light.

Q 7. Laws of refraction
Or
Snell’s law

*Refraction of light follows the following two laws:
Ist law: The incident ray, the refracted ray and the normal all lie on the same plane.
2nd law: it is defined as the ratio of sine of incidence angle <i to the sign of angle of refraction <r  is constant for the given pair of medium i,e
Sin i / Sin r = Constant

Q. Refractive index:
It is defined as the ratio of speed of light in vacuum to the speed of light in that medium. It is denoted by n.

Speed of light in vaccum ÷ Speed of light in given medium.

Q. Magnification:
It is defined as the ratio of height of an image to the height of an object it is denoted by m.
Height of an image ÷ Height of an object
m= h’/h.  Or. v/u
v, h’ is height of an image
u, h is height of an object.

Real image : When the rays of light after reflection from a mirror actually meet at a point, then the image formed by these rays is said to be real. Real images can be obtained on a screen.

Virtual image: When the rays of light, after reflection from a mirror appear to meet at a point then the image formed by these rays is said to be virtual. Virtual images can’t be obtained on a screen.

Define Lens:  The transparent refracting medium bounded by two surfaces in which at least one surface is curved is called lens.

Sign Conventions of Spherical Mirror :

All the distances are measured from the pole of the mirror as the origin.

Distances measured in the direction of incident rays are taken as positive.

Distances measured opposite to the direction of incident rays are taken as negative.

Distances measured upward and perpendicular to the principal axis are taken as positive.

Distances measured downward and perpendicular to the principal axis are taken as negative.

Parameters of Mirror:

Centre of curvature : The centre of hollow sphere of which mirror is a part.

Radius of curvature: The radius of hollow sphere of which mirror is a part.

Pole: The centre of mirror (middle point) is pole.

Principal Axis : The line joining the pole and center of curvature is called principal axis.

Aperture: Size of mirror is called aperture of mirror.

Principal Focus: The point on the principal axis, where all the incident rays parallel to principal axis converge or diverge after reflection through mirror.

Focal Length: The distance between pole and focus point is focal length.

1. Define the principal focus of a concave mirror.

Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is called the principal focus of the concave mirror.

2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Radius of curvature (R) = 20 cm

Radius of curvature of the spherical mirror = 2 × Focal length (f)

R = 2f

f= R/2 = 20 / 2 = 10

Therefore, the focal length of the spherical mirror is 10 cm.

3. Name the mirror that can give an erect and enlarged image of an object.

The mirror that can give an erect and enlarged image of an object is Concave Mirror.

4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Convex mirror is preferred as a rear-view mirror in cars and vehicles as it gives a wider field of view, which helps the driver to see most of the traffic behind him. Convex mirrors always form an erect, virtual, and diminished image of the objects placed in front of it.

1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Radius of curvature (R) = 32 cm

Radius of curvature = 2 × Focal length (f)

R= 2f

f = R/2 = 32/2 = 16

Therefore, the focal length of the given convex mirror is 16 cm.

2. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?

Magnification produced by a spherical mirror:

Object distance (u) = – 10 cm

v = 3 × (- 10) = – 30 cm

Therefore, the negative sign indicates that an inverted image is formed in front of the given concave mirror at a distance of 30 cm.

1. A ray of light travelling in air enters obliquely into water. Does the light ray bends towards the normal or away from the normal? Why?

The light ray bends towards the normal. When a light ray enters from an optically rarer medium (which has low refractive index) to an optically denser medium (which has a high refractive index), its speed slows down and bends towards the normal. As water is optically denser than air, a ray of light entering from air into water will bend towards the normal.

2. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 x 108 ms-1.

Refractive index of a medium (nm) = Speed of light in vacuum/Speed of light in the medium

Speed of light in vacuum (c) = 3 × 108 m/s

Refractive index of glass (ng) = 1.50

Speed of light in the glass (v) = Speed of light in vacuum/ Refractive index of glass

= c/ng

=3 × 108/1.50 = 2x 108 ms-1.

5. The refractive index of diamond is 2.42. What is the meaning of this statement?

Diamond has a refractive index of 2.42 which means that the speed of light in diamond will reduce by a factor of 2.42 as compared to its speed in the air.

In other words, the speed of light in diamond is 1/2.42 times the speed of light in vacuum.

1. Define 1 dioptre of power of a lens.

Dioptre is the SI unit of power of lens is denoted by the letter D. 1 dioptre can be defined as the power of a lens of focal length 1 metre.

2. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.

The position of the image should be at 2F since the image is the real and same size.

It is given that the image of the needle is formed at a distance of 50 cm from the convex lens. Therefore, the needle is placed in front of the lens at a distance of 50 cm.

Object distance (u) = – 50 cm

Image distance, (v) = 50 cm

Focal length = f

According to the lens formula,

3. Find the power of a concave lens of focal length 2 m.