Quiz 2: Sunday 11th October ( Refraction & Diffraction) Test 1: Sunday 18th October (Chapter 13 & 14)-All

REFRACTION Is the bending(change of direction) of light when it passes from one material into another. The speed of light decreases when light enters a dense medium. Refracted ray bends towards the normal.

The pencil appears broken when in water. The rays reaching our eyes bend at the surface. The beam bends when it enters glass The fish appears near the surface of water that it real is. Rays from the fish refract at the surface.

Coin appears to be near the surface of water Coin in Water Coin appears to be near the surface of water Apparent depth Real depth Glass of water

Glass to Air Air to glass (Less dense to dense medium) Glass to Air (Dense to less dense) Speed of light decreases and refracted ray Speed of light increase and refracted ray Bends Towards the Normal Bends Away from the Normal A normal is a line drawn 90o to the surface at the point where the light enters the medium.

Refraction by glass block Exit ray Ray Box Glass Block Incident light

Complete the paragraph below Refraction is caused by the change of ____________of light when it moves from one material to another. When light moves from less dense to dense material the speed of light____________and the refracted ray bends_______________the normal. Also, when light moves from dense to less dense material the speed of light____________. The refracted ray bends______________ the normal. The ratio of speed of light in a material to the speed of light in a vacuum is called__________________ of the material.

Boundary Medium 1 Medium 2 Refracted ray Medium 1 Medium 2 Incident ray Which medium is denser. Explain your answer.

TEACHING POINT Plot a graph of sin i against sin r. Define refractive index of a material.

The gradient of the graph gives the refractive index(n) A graph of sini against sinr is a straight line through the origin. sini sinr The gradient of the graph gives the refractive index(n)

Draw graph of sin i (y- axis) against sin r (x-axis) Complete the table for sin i and sin r. i/ o r / o 30 20 35o 23o 40o 26o 45o 29o 50o 32o 55o 34o 60o 36o sin i sin r 0.50 0.34 0.57 0.39 0.64 0.44 0.70 0.49 0.76 0.53 0.81 0.56 0.86 0.59 Draw graph of sin i (y- axis) against sin r (x-axis)

Draw graph of sin i (y-axis) against sin r (x- axis) (0.68, 1.0) Slope = n (0.14, 0.2) Refractive index (n) = = 1.48

When the incident angle is 900, no refraction occurs When the incident angle is 900, no refraction occurs. The rays of light goes straight unchanged

Class work: 1. Write down a formula for refractive index of a material given: (i) relative speeds (ii) Relative angles (iii) Page 187#13.14-13.16 2. Explain what is meant by (i) Monochromatic light (ii) Critical angle (iii) Total internal reflection (iv) State two conditions for total internal reflection to occur. (V) Page 190#13.19-13.22 3. State one use of total internal reflection 4. Draw a diagram of an optical fibre to show how a signal is transmitted

REFRACTIVE INDEX(n) Refractive index is the ratio of speed of light in a vacuum to the speed of light in a material.   The speed of light in glass 2x108 m/s. Calculate the refractive index of glass if the speed of light in air is 3x108 m/s 𝑛= 3𝑥 10 8 2𝑥 10 8 n=1.5 (No units)

Exercise 1. Speed of light in 3 x 108 m/s   Exercise 1. Speed of light in 3 x 108 m/s The speed of light a material is 2.5 x 108 m/s. Find the refractive index of materials. The refractive index of crown glass is 1.4. Find the speed of light in crown glass. Find the speed of light in a water of refractive index 4/3.

Angle of incidence Snell’s Law Normal i Incident ray The ratio of the sine of angle of incidence to the sine of angle of refraction is a constant called refractive index. Air Glass r Refracted ray   Angle of refraction

Glass to air Air to glass    

300 find refractive index 𝑛= 0.500 0.342 = 1.46 20o Air Glass   Normal 300 Incident ray Air Glass Refracted ray 𝑛= sin 30 sin 20 = 𝑛= 0.500 0.342 = 1.46 20o

Find r. 600 n= 1.5 r Angles are always given to 1 decimal place Air Normal   600 Incident ray Air Glass n= 1.5 1.5= 𝑠𝑖𝑛60 𝑠𝑖𝑛𝑟 Refracted ray r sin 𝑟 = 𝑠𝑖𝑛60 1.5 = 0.577 r = sin-1(0.577) = 35.20 Angles are always given to 1 decimal place

CRITICAL ANGLE(C): The critical of glass is 420. The angle of incidence which gives angle of refraction is 900 when light is travelling from dense to less dense medium Or: Minimum angle at which total internal reflection occurs Watch video Click Link The critical of glass is 420.

MONOCHROMATIC LIGHT Light of a single wavelength Example : Laser light

TOTAL INTERNAL REFLECTION(T.I.R) Complete reflection of light when angle of incidence is greater than critical angle when light is traveling from dense to less dense. This occurs when: Light is travelling from dense to less dense medium. The angle of incidence is greater than critical angle.

Total internal reflection Watch Video

Calculating Critical Angle It is possible to calculate the refractive index using the critical angle r= 90° Air Glass c r = 900 i = C n

Calculate the critical angle of a glass of refractive index of 1.5 Calculate the critical angle of a water of refractive index of 1.3. 2Calculate the refractive index of a ,material whose critical angle is 470.

Light from air to glass at 90o. No Refraction A & D Critical angle of glass in prism 42o. Draw the path taken by light shown below. Light from air to glass at 90o. No Refraction A & D B 450 A The angle of incidence of 450 greater than critical angle of 420 Total internal reflection occurs. B & C 450 C D

Critical angle of glass in prism 42o Light from air to glass at 90o No Refraction Light attempting to go from glass to air but angle of incidence greater than critical angle. 450 Total Internal Reflection Light from glass to air at 90o No Refraction

1. Reflective road signs. Applications of Total Internal Reflection 2. Optical Fibres used in -communication -Endoscopes -Periscopes Advantages of optical fibres over copper conductors Less interference. cheaper raw material occupy less space / more information (carried) in the same space flexible for inaccessible places/ do not corrode

Optic Fibre Glass cladding of low refractive index Glass core of high refractive index

Total internal; Reflection in Optic fibre Glass of low refractive index N N Normal

Visible Spectrum White light is made up of the seven colours of the rainbow. Red, orange, yellow, green, blue, indigo, and violet.

Dispersion The splitting of white light into seven colours by a prism Least refracted Most refracted

RAINBOWS We see rainbows because the light is first refracted by raindrops and then reflected off the back of the raindrop.

RESOURCES Click on the link below Sample questions on Refraction of waves. Sample questions on Diffraction Sample question on refraction of light