Nature of Light
Laws of Reflection.  Rough surface Fig.7. Regular Reflection. Fig.8. Irregular Reflection. (diffuse reflection) In fig.8 the reflected rays are shown in red. The two Laws of reflection.
Refraction of light
 The ratio is called the refractive index.
The refractive index of water is 1.3 whereas that of glass is 1.5. Diamond 2.4 Vacuum 1.000 Air 1.003 Fig.12. A ray of light refracting by glass and emerging out is shown here. A ray of light cannot be seen but if it goes along a white paper it can be seen as the illuminated region. Dispersion of white lightWe have given the name white light to the pure sun light we get after dawn and before dusk. At sun set and sun rise we do not get white light. There are many ways to show that sunlight is made uo of several colours. Easiest way is to look at the rainbow. Passing the light through a prism van easily demonstrated as shown here.  Fig.13. Dispersion of sun liht producing a continuous spectrum. why do we see some objects coloured even with white light ? The colours that materials reflect vary. A cherry absorbs all colours except red. What we see is the reflected colour or a mixture of colours to which we give various names.  Fig. 14. Objects absorb many colours and reflect a few. Light as wavesLight energy may be taken as particles, which are called photons or waves. Tje frequency and wave length of different bcolours is shown here.  Fig.15. Wave properties of colours. Total Internal Reflection.  b Rays are coming up in a tank of water. Numbers 1 and 2 undergo refraction only. 3 shows some reflection also. Number 4 gives the highest possible reflection. What happens if you increase the angle of incidence further ? There will be no refraction and there will only be reflection. 48 degrees is the critical angle for water to air. Finding the Critical angle. For water the Critical angle is the refracted angle when the angle of incidence is 90 deg. Example 1 Calculate the angle of refraction when a ray is incident on water making an angle of incidence equal to 30 deg. Sin i. / sine r = 1.3 sine i. - sin 30 - 0.5 0.5 / sine r = 1.3 sine r = 0.5/1.3 = 0.3846 r = 22.6 deg. Example 2. Calculate the critical angle for glass whose refractive index is 1.5. Sine i. / sine r = 1.5 sine i. = 90 deg. = 1.0 1/ sin r = 1.5 sine r. = 1/ 1.5 = 0.6666 r = 41.8 deg.
Wave length is in nano meters. Blue can get scattered about 9 times more than the red as they have a shorter wave length. For specialised sections click :- Light Lenses. :- Sense of Vision.
Q. 1.0 Which theory explains the phenomenon in column one.
10 Marks
Q 2.0 How can you describe what mainly happens to light on following occasions?
1 Entering clear water making an angle of 10˚. 2 Candle light incident on a wall. 3 A diver in a pool looking up and sees the bottom. 4 One sees his face through a mirror. 5 Traveling a great distance into the atmosphere? 3 x 5 = 15 Marks Q.3.0
1 According to the law of reflection if a ray makes an incident angle of 42˚ what will be the reflected angle? 2 In order to see a flower what must be formed on the retina of your eye? 3 What should be the angle of incidence of a ray to enter the water just to prevent refraction and to minimise reflection? 4 What colours are mostly absorbed by the green leaves for photosynthesis? 5 What colour in the visible spectrum can travel the greatest distance in the atmosphere? 3 x 5 = 15 Marks
2 x 10 = 20 Marks
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