 # Light - Refraction Sec 3 Physics For E-Learning, please read slides 1 to 10.

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Light - Refraction Sec 3 Physics For E-Learning, please read slides 1 to 10

Refraction Refraction is the bending of light when it enters from one transparent medium into another. As a light ray travels from one medium into another medium where its speed is different, the light ray will change its direction unless it travels along the normal. When light slows down when it moves from medium A to medium B, we say that the medium B is optically denser than medium A.

Using a laser pen, a beam of light is sent from water into air. laser pointer When light strikes such transparent boundary, both reflection and refraction occur. Refraction of Light water What do you think will happen to its path of travel as it strikes the air-water boundary? reflection refraction air glass reflection refraction reflection

Glass is optically denser than water hence light bends more when it enters glass compared to water.

Refraction Terminology

Direction of bending of light When light enters an optically less dense medium, the light ray bends away from the normal. When light enters an optically denser medium, the light ray bends toward the normal. air bends away from the normal bends towards the normal air water

air glass air glass i r from an optically less dense medium to an optically denser medium, i > r from an optically denser medium to an optically less denser medium, i < r bends towards the normal bends away from the normal i r Direction of bending of light

Refraction of Light air glass The more optically dense the material, the slower the speed of light in that material Air has lower optical density, light travels faster. Glass has higher optical density – light travels slower, bend towards normal Air has lower optical density, light travels faster, bend away from normal. The change in speed at the transparent boundary between two media causes light to change direction. Optical density of glass > water > air > vacuum

Refractive Index, n air glass c v c For example, the refractive index of glass at the air-glass boundary is given as: c n = The refractive index of a medium is the ratio of the speed of light in one medium relative to the speed of light in the other medium. where c and v are the speed of light in air and glass respectively. v

Refractive Index of Various Medium

Laws of Refraction The incident ray, the refracted ray and the normal all lie on the same plane. For two particular media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant (Snell’s Law) sin i = constant sin r

air glass  2 2  1 1  1 1  2 2 The ratio of the two sin  s gives the same refractive index, and this relationship is called Snell’s Law. sin  1 sin  2 n =n = Then: if  1 represents the angle of incidence in the less dense medium, and  2 represents the angle of refraction in the denser medium; Snell’s Law

Refractive Index When light passes from vacuum (or air) into a given medium (eg. water), the constant ratio of is known as the refractive index, n, for that medium. Angle of incidence Angle of refraction

A ray of light approaches a glass-air boundary at an angle of incidence i = 30 . What is the refractive index of the glass if the angle of refraction r = 49  ? air glass 30  49  sin  1 sin  2 n = sin 49  sin 30  = = 1.5 Sample Problem 1 angle in less dense medium

water 35  air A ray of light approaches a water-air boundary at an angle of incident i = 35 . (i)Complete the path of the light as it crosses the water-air boundary. (ii) What is the angle of refraction if the refractive index of water n = 1.3? sin  1 sin  2 n = sin 35  sin r = 1.3 = 26  r Sample Problem 2 r

Daily Phenomena of Refraction Swimming pool and ponds appear shallower than it really is. Object is at a deeper depth than where it appears to be. Bent objects in liquids

The picture below be easily reproduced with a laser pointer and a transparent semi- circle glass block in a darkened room. As light enters the glass block, it bends at the surface instead of traveling its original path. What do you think will happen if the angle of incidence in the glass block is increased gradually? Total Internal Reflection

A light ray from water is incident on the water - air boundary. The angle of incidence is gradually increased. (i)Calculate the angle of refraction if the refractive index of water is 1.3. (ii) What do you observe about the angle of refraction as the angle of incidence is gradually increased? 57 o air glass 40 o air glass 30 o 41 o 20 o air glass 26 o What is the maximum angle of refraction that can be produced? Total Internal Reflection As the angle of incidence increases, angle of refraction increases as well.

At a certain critical angle c, a maximum angle of refraction = 90  is produced. A weak reflected ray is also produced. If the next incident angle is greater than c, no more refraction will occur. The reflected ray becomes very strong and intense. 90  air glass c air glass i> c r if incident angle > critical angle:  no light ray from the optically denser medium will be refracted.  all light rays will be totally internally reflected into the optically denser medium. c weak reflection strong reflection Total Internal Reflection

Critical angle and Total Internal Reflection Total internal reflection only occurs when:  a light ray is travelling from an optically denser medium to an optically less dense medium. Critical angle c is defined as the angle of incidence from a denser medium which produces an angle of refraction of 90 . 90  air glass c c weak reflection air glass i> c r strong reflection  the angle of incidence is greater than the critical angle.

Critical Angle Critical angle is the angle of incidence in the optically denser medium for which the angle of refraction in the less dense medium is 90 o. Refractive index Critical angle

A laparoscope is a medical equipment inside a hollow, thin tube. It is connected to a camera and a high intensity light for doctor to see the structure inside our body. Perform an internet search to find out how total internal reflection plays a part in different fields. Be prepared to share with your classmates what you have learnt from your research next week. Total Internal Reflection Enrichment

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