Refraction.docx

**Refraction of light** 1. Is glass denser or rarer medium as compared to air? 2. What will happen to the velocity of light ray when it passes from glass to air? 3. What happens to the light when it is incident normally on the surface of the glass slab? 4. Copy and complete the follo...

**Refraction of light** 1. Is glass denser or rarer medium as compared to air? 2. What will happen to the velocity of light ray when it passes from glass to air? 3. What happens to the light when it is incident normally on the surface of the glass slab? 4. Copy and complete the following. B. **Refraction of light through a glass slab** C. **Laws of Refraction** 1. In Figure 5.2, the incident ray lies on plane WXYZ. The refracted ray BC, normal NM and normal N~1~M~1~ and the emergent ray CD can never be on a different plane. So, the first law of refraction states that the incident ray, the refracted ray, emergent ray and the normal lie on the same plane. 2. The refractive index of a medium is calculated by comparing the velocity of light in the first medium to the velocity of light in second medium. But, [ *EM*] = Sin i (sine function of angle of incidence) and [ *NF*] = Sin r (sine function of angle of refraction) ##### Refractive Index *2* -- -- -- -- ##### Principle of reversibility *\` 1n2 =* - Place a glass slab on a white sheet of paper fixed on a drawing - Remove the slab and draw a normal (perpendicular) NN', on one of the longer sides of its outline. - With the help of a protractor, draw angles of incidence of 40˚,50˚,and 60˚ to the normal in the air as shown in Figure 5.4. - Fix three pins P1, P2 and P3 on the incident ray, making an angle of - Place the glass slab on its outline. Look through the other side of the slab, opposite to the incident ray, and place the other three pins - Remove the slab and join the three points P4, P5 and P6 with a straight line. Extend the line to meet the outline PQRS at O~2~. - Join O1 to O2 to represent the refracted ray for + i = 40˚. Then - Repeat Step 4 to Step 7, for angles of incidence of 50˚and 60˚. - Calculate the sine of angles of incidence and the sine of angles of refraction with the help of a calculator,and record them in the Table -- -- -- -- -- -- -- -- -- -- -- -- 1. What happens to the angle of refraction when the angle of incidence increases? 2. How do the change in the values of + i and + r affect the refractive index? What can you conclude? 3. What conclusion can you draw from the ratios of *sin i* to *sin r* in all the three sets of readings? What could be the reason for this? ##### Real and apparent depth - Place a glass slab on a white sheet of paper fixed on a drawing - Remove the slab and draw a normal (perpendicular) NN', on one of the longer sides of its outline. Draw an incident ray inclined as shown - Replace the slab in its outline. Look through the side of the slab - Remove the slab and join the points P , P and P with a straight line. - Draw another incident ray on the same side and incident at the same - Repeat steps 3 and 4 to get the second emergent ray. - Remove the slab and extend the two emergent rays towards the incident rays. Mark point O/ at the point of intersection of the two - Measure the perpendicular distance AO (real depth) and AO' (apparent depth). Calculate the refractive index of the glass slab. D. **Dispersion of light** -- -- -- -- -- -- -- -- The ratio of wavelength of light in air to wavelength of light in that medium is E. **Total internal reflection** 1. Light must pass from optically denser medium to optically rarer medium. 2. The angle of incidence in the denser medium must be greater than the critical angle for the given pair of media. ##### Natural phenomena due to total internal reflection #### Mirage #### Sparkling of diamond  ##### Applications of Total internal reflection #### Optical fibre #### Periscope 1. Tshering passes laser light from air to glass and is incident on a glass surface at an angle of 30˚. At what angle will it get refracted inside the glass? Take absolute refractive index of glass as 1.5. 2. Draw a diagram of a periscope that is constructed using plane mirrors. 3. A ray of blue light and a ray of orange light are passing through water in a beaker. Which light reaches the bottom of the beaker first? Which light do you think will have higher refractive index?