Light continued

Refraction of light is the bending of light as it goes from one optical medium to another Less dense to more dense: bends towards normal More dense to less dense: bends away from normal

A medium is

Index of Refraction

Incident ray i r Refracted ray Glass block

The Laws of Refraction of Light 1. The incident ray, the normal and the refracted ray all lie in the same plane 2. where n is a constant This is called Snell’s Law

When light travels from a rarer to a denser medium it is refracted towards the normal e.g. air to water

when light travels from a denser to a rarer medium it is refracted away from the normal e.g. glass to air

Experiment to Verify Snell’s Law and determine the refractive index of glass

i 1 2 r Block of glass 3 4

Experiment to Verify Snell’s Law and determine the refractive index of glass Method Outline the glass block on paper Stick pins 1 and 2 in the paper in front of the block Stick pins 3 and 4 in line with the images of 1 and 2, as seen through the block

4. Remove the pins, join the pinholes and draw the normal 5. Measure angles i and r 6. Repeat for different angles of incidence 7. Draw a graph of sin i vs. sin r 8. The slope of the graph is the refractive index of glass

Result i r Sin i Sin r Sin i/Sin r

Sin i Sin r

Slope of graph = Thus the refractive index of glass is ____

Conclusion The graph is a straight line graph through the origin, thus verifying Snell’s Law The slope of the graph gave the refractive index of glass

Real and Apparent Depth

A swimming pool appears to be less deep than it actually is, due to refraction at the surface of the water We can calculate the refractive index of a liquid by using n =

Refractive index n =

Question 1 When light passes from air into a liquid, the angle of incidence and refraction are 57˚and 30˚ respectively. Calculate the refractive index of the liquid. Answer Formula:

Question 2 A ray of light is incident at 35˚ on (a) a glass surface and (b) a water surface. Calculate the angle of refraction given that, ng=1.52 nw=1.33 Answer Formula:

Textbook Examples Questions 1-3 on page 31

Total Internal Reflection This may occur when light goes from a denser to a less dense medium As i is increased so is r Eventually r = 90˚ At this point i has reached the ‘critical angle’

r = 90˚ i = critical angle

Total Internal Reflection!!!

It is reflected back into the first medium If i is increased beyond the critical angle, the ray does not enter the second medium It is reflected back into the first medium We can also find the refractive index of a material using n = C = critical angle

Example The critical angle of glass is 41.81˚ Find the refractive index of glass n = n = 1/0.666 n = 1.5

Refractive index n =

Example The refractive index of glass is 1.5 This value is for a ray of light travelling from air into glass So ang = 1.5 = Or gna = =

Applications of Total Internal Reflection Periscopes (using a prism) Diamonds and bicycle reflectors Optical fibres – in telecommunications and by doctors

Prisms Prisms are considered to be better than mirrors for reflecting light. This is because they use total internal reflection. This is the reason prisms are used in periscopes and binoculars.

Optical Fibres Total Internal Reflection has given rise to the use of optical fibres. The signal is fired at an angle greater than the critical angle so that it is reflected all the way along the fibre. The light is therefore trapped within the fibre. This technology is used to transmit information over long distances.

Endoscopes These are used in medicine. They contain two bundles of optical fibres, one to carry light to the organ being viewed and the other to carry the image of the organ back to the operator.

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

Glass of high refractive index Glass of low refractive index N N Normal

Remember that rays are path-reversible AIR GLASS A B

Mirages Mirages are caused by the refraction of light in air due to temperature variations

SKY

Convex lens (converging) LENSES Made of transparent material, usually glass Convex lens (converging) Thick in centre and thin at edges

Concave lens (diverging) Thin in the centre and thick at the edges

Ray diagrams for lenses 1. Ray incident parallel to principal axis is reflected out through focus 2. Ray incident through focus is reflected out parallel to axis 3. Ray incident through optic centre continues in straight line

RAY DIAGRAMS FOR CONVEX LENSES Optic centre

RAY DIAGRAMS FOR CONCAVE LENSES Optic centre

Geometric Optics 2.03

The image in a concave lens is always virtual, erect, and diminished Concave lens always has negative f and v

YouTube - 3D Medical Animation: LASIK Eye Surgery for Nearsightedness

Lens formulae m =

Example An object is placed 10cm away from a convex lens of focal length 12 cm. Calculate the nature, position and magnification of the image.

f = 12 u = 10 v = ? v = -60cm

m = m = = 6 So the image is formed 60cm from the lens It is virtual Magnification is 6

Example 2 An object is placed 40cm from a concave lens of focal length 50cm. Find the position, nature and magnification of the image.

u = 40cm f = -50cm v = ?? v = -22.2 cm

m = m = = 0.56 So the image is formed 22.2cm from the lens It is virtual Magnification is 0.56

Experiment to Measure the Focal Length of a Convex Lens Pin Plane mirror

Experiment to Measure the Focal Length of a Convex Lens Method A rough estimate of the focal length may first be obtained by focusing the image of a distant object on a sheet of paper Set up the apparatus as in the diagram Move the pin in and out until there is no parallax between the pin and its image Measure the distance from the pin to the centre of the lens. This is the focal length

Result The distance from the pin to the centre of the lens was ______ Conclusion The focal length of the lens is ______

Experiment to Measure the Focal Length of a Concave Lens Convex lens Pin F Plane mirror

Method Set up the apparatus as in the diagram The focal length of the convex lens is first measured The concave lens is combined with the convex lens (of shorter focal length), and so overall it behaves as a convex lens Move the pin in and out until there is no parallax between the pin and its image Measure the distance from the pin to the centre of the lens. This is the focal length of the combination

Results The focal length of the convex lens is 22cm The focal length of the combination is 51cm. The focal length of the concave lens can be calculated from the formula Where F = focal length of combination f1 = focal length of convex lens f2 = focal length of concave lens

F = 51cm f1 = 22 f2 = ??

Conclusion The focal length of the concave lens is 38.69cm