Sound waves speed up when they enter water. In air, the speed is about $330 \mathrm{m} {\mathrm{s}}^{-1}$, but in water the sound waves travel at about $1500 \mathrm{m} {\mathrm{s}}^{-1}$. Explain whether total internal reflection will occur when sound travels from air into water and from water into air.  Explain how this affects how well you can hear sounds above the surface of the water if you are under the surface. 

Light enters an optical fiber at an angle of incidence of ${15}^{\circ }$. 

On a copy of this drawing, draw a line to show how the path of the light continues down the fiber.

Light enters an optical fiber at an angle of ${15}^{\circ }$.

The refractive index of the fiber is $1.4$. Calculate the angle of refraction of the light beam shown entering the fibre.

Light enters an optical fiber at an angle of ${15}^{\circ }$.

The refractive index of the fiber is $1.4$. Calculate the critical angle for this optical fiber.

Light enters an optical fiber at an angle of ${15}^{\circ }$.

The refractive index of the fiber is $1.4$. Calculate the speed of light in the fibre.

When designing an optical fiber, scientists need to consider the effects of temperature on the refractive index. As an initial experiment, they measure the refractive index of water at different temperatures. Suggest what the independent and dependent variables for their experiment should be.

When designing an optical fiber, scientists need to consider the effects of temperature on the refractive index. As an initial experiment, they measure the refractive index of water at different temperatures. Suggest what the independent and dependent variables for their experiment should be. The scientists use a laser pen and a rectangular tank of water. They take a photograph of the tank from above. 

Give the names of two pieces of measuring equipment that the scientists will need.