Light - Refraction

Light - Refraction

LEARNING OBJECTIVES

• Apply the mirror formula to solve numericals.

• Classify optical medium as a rarer or a denser medium.

• Comprehend definition of refraction of light and represent it diagrammatically

• Evolve laws of refraction of light, verify them experimentally.

• Analyse the reason behind every day phenomena using knowledge of

  refraction of light.

• Interpret the meaning of the term refractive index and its relation to the velocity of light.

Multiple Choice

An object is kept at a distance of 4 m in front of a spherical mirror, which forms its erect image at a distance of 1 m from the mirror. What is the magnification?

• 1/4

• 4

• -1/4

• -4

Multiple Choice

An object is kept at a distance of 4 m in front of a spherical mirror, which forms its erect image at a distance of 1 m from the mirror. What type of mirror is this?

• concave mirror

• convex mirror

• plane mirror

Multiple Select

A spherical mirror produces a magnification of -1 on a screen placed at a distance of 50 cm from the mirror

1) Mention the type of the mirror.

• 1)The mirror is concave since it forms a virtual image.

• 2)The mirror is concave since it forms a real image.

• 1)The mirror is convex since it forms a real image.

Multiple Choice

A spherical mirror produces a magnification of -1 on a screen placed at a distance of 50 cm from the mirror. Find the distance of the image from the object.

• Zero

• 50 cm

• 25 cm

Multiple Choice

A spherical mirror produces a magnification of -1 on a screen placed at a distance of 50 cm from the mirror. What is the focal length of the mirror?

• 50 cm

• 25 cm

• 0

Ray diagram when u = v = R = 50 cm and f = 25 cm

Magic arrow activity

Material required: A glass, Water, A note card, A marker

What to Do

Fill your glass of water. Draw a horizontal arrow on a note card. Put the note card behind the glass of water and slowly move the note card back. Look through the glass from the front and observe the arrow. What appears to happen to it?

Open Ended

Why is arrow reversed in direction?

Pencil and water activity

Dip a pencil in the glass full of water. What do you observe?

Open Ended

Why do you think the pencil bends when dipped in water?

LAWS OF REFRACTION

First Law of refraction of light

I)  The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.

  

Second Law of refraction of light

I)  The ratio of the sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given color and for the given pair of media.      ( This law is also known as Snell's law of refraction.) 

NCERT INTEXT QUESTIONS

Q. A ray of light travelling in air enters obliquely into water.

Does the light ray bend towards the normal or away from the normal? Why?

When light enters from air to water

The light ray bends towards the normal. Since water is optically denser than air, a ray of light travelling from air into the water will bend towards the normal. 

Open Ended

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 × 10⁸ m s⁻¹.

Open Ended

Among Kerosene, turpentine and water, speed of light will be fastest in which of them?

Open Ended

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

The refractive index of diamond is 2.42. 

This suggests that the ratio of speed of light in air to speed of light in diamond is 2.42

Key points of Today's Lesson

• Refraction of Light: The bending of light at the interface of two different mediums is called Refraction of light.

• If the velocity of light in medium is more, then medium is called optical rarer. Example, air or vacuum is more optical rarer.

• If the velocity of light in medium is less, then medium is called optical denser.Example, glass is more denser than air.

Refractive Index: It represents the amount or extent of bending of light when it passes from one medium to another.

• Absolute refractive index of medium = Speed of light in air (c) / Speed of light in medium(V_m)

• Relative Refractive Index.

  Refractive index of medium 1 with respect to medium 2 = Speed of light in medium 2 (V₂) / Speed of light in medium 1 (V1)

Law of Refraction: 

• “The incident ray, refracted ray and normal at the point of incidence all lie in the same plane.”

• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.”

  sin i /sin r = constant (n)

Homework

Complete the assignment sheet Light Assignment-2 uploaded in assignment tab and submit it.

Revise all the topics discussed today in class.