Hi Nicole!

Q40: I've always found lens and prism things hard to understand without being able to first SEE them.

Here's my favourite video for refraction and reflection:

https://www.youtube.com/watch?v=NAaHPRsveJk

First, you can see the light beam is refracting through the prism. It enters at a "low angle" and exits at a "high angle", indicating that nglass > nair. Light will bend TOWARDS things that have a high n and AWAY from things that have a low n.

At some point, if you increase the incident angle too far, NONE OF THE LIGHT CAN PASS THROUGH the prism! That's called the critical angle. It is unique for every PAIR of materials, and can be found from

sin Θc = n2 / n1

1 is the material that the light starts in (glass)
2 is the material that the light wants to get to but can't (air)

In the question, n1 = nprism and n2 = ncell. We can assume that the cell is filled with air or water. nprism > ncell and total internal reflection occurs.

Q47: Here, they're talking specifically about the fraction of white light that reaches your eye from the sun WHEN YOU LOOK AT THE SKY. Based on the passage, visible light is scattered by particles IN THE SKY. The passage says: the sky appears blue. So visible light, which is made up of the colours of the rainbow, must be getting scattered, except for the blue part. Why is THE SKY blue? Only the blue light reaches you when you look at THE SKY.

I'm emphasizing sky here because we are only talking about why the sky is blue, not other objects. We only care about the sky in this question. As you see when you look around you... not everything is blue.