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# 5.2 Refraction

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### 5.2 Refraction

1. 1. (denser) (less dense) 5.2 Refraction of Light When light travels through two materials of different optical densities , it will change direction at the boundary between them. In other words, it changes speed and bends . How is light refracted ?
2. 2. i r Normal Incident ray Angle of incidence refracted ray Angle of refraction Medium 1 Medium 2 Angle between incident ray and normal Line perpendicular to the boundary Angle between refracted ray and normal Original path of light ray Important Terminologies
3. 3. Bending of the Light Normal Air Glass Normal Air Glass Light is not bent when it travels along the normal. Light bends towards normal when it enters an optically denser medium . However, light bends away from normal when it enters an optically less dense medium . Bends towards the normal Bends away from the normal
4. 4. Refractive Index Normal Air Glass i r p q <ul><li>When light travels between two medium with different optical densities , it changes speed and bends. </li></ul><ul><li>The angle of bending of light depends on the </li></ul><ul><li>(i) refractive index of the mediums, </li></ul><ul><li>(ii) angle of incidence, i . </li></ul><ul><li>3. Refractive index, n = </li></ul>
5. 5. Example 1 <ul><li>The speed of light in glass is 2.0 x 10 8 m s -1 and the speed of light in air is 3.0 x 10 8 m s -1 . Calculate the refractive index of the glass. </li></ul><ul><li>(b) The refractive index of water is 1.3. What is the speed of light in water if its speed in air is 3.0 x 10 8 m s -1 . </li></ul>v = 2.3 x 10 8 m s -1 Solution (a) (b)
6. 6. The Law of Refraction – Snell’s Law <ul><li>The incident ray , the refracted ray and the normal at the point of incidence all lie in the same plane. </li></ul><ul><li>The ratio of is a constant , where i is the angle of incidence and r is the angle of refraction. </li></ul><ul><li>Refractive index , </li></ul>Normal Air Glass i r p q
7. 7. Example 2 (a) Air Crown glass Normal 30 o r Calculate the angle of refraction in crown glass. n =1.52 (b) Air water Normal 60 o i Calculate the angle of incidence in water. n =1.33 Solution (a) (b)
8. 8. Real depth, H and apparent depth, h The legs appear to be shorter than their actual length in a pool. Why ?
9. 9. RAY DIAGRAM Normal 1. Draw the normal line. <ul><li>Mark the position of image. </li></ul>3. Draw two diverging lines from image. 4. Connect ray line from surface of water to object. h = apparent depth , H = real depth h H
10. 10. Relationship between real depth, H and apparent depth, h Refractive index , Apparent depth, h / cm Real depth, H / cm O
11. 11. Example 3 h 10 cm Solution <ul><li>Why does a stick partially immersed in water appear bent ? </li></ul><ul><li>Complete the ray diagram in the figure to show how the stick appears bent in the water. </li></ul><ul><li>Calculate the apparent depth, h if given the refractive index of water is 1.33. </li></ul><ul><li>This is due to the refraction of light. </li></ul>(b) (c) Refractive index,
12. 12. Summary <ul><li>When light travels between two medium with different optical densities , it changes speed and bends . </li></ul>Refractive Index, n
13. 13. Evaluation ~ Challenge Yourself ( Pg 146 ) <ul><li>If the speed of light in air is 3.0 x 10 8 ms -1 and the refractive index of diamond is 2.42, what is the speed of light in diamond? </li></ul>Homework : Mastery Practice 5.2 (Pg 145) and Challenge Yourself, Q3 (Pg 146) 2. If light is directed through a piece of flint glass with refractive index 1.62, at an angle of incidence 30 o , what is the angle of refraction? Solution