Light

Light from the Sun crosses roughly $150$ million kilometres of virtually empty space to reach Earth.

1. a) Does light need a material medium to travel through?
2. b) What are waves called that, like light, can travel through a vacuum?
3. c) If a distant star were to explode, what would we perceive from Earth: its light, its bang, or both? Justify your answer.

The speed of light in a vacuum is $c = 3\cdot10^{8}\ \text{m/s}$.

1. a) What distance does light travel in one second? Express it in metres and in kilometres.
2. b) The Earth's circumference is about $40\,000\ \text{km}$: how many times could light travel around the planet in one second?

On passing through a prism, white sunlight splits into a band of colours like a rainbow.

1. a) Name the colours of this band in order.
2. b) What invisible radiation exists beyond red and beyond violet? Give an example of an application for each.
3. c) Do red light and violet light travel at different speeds in a vacuum?

Sunlight takes about $8\ \text{min}$ and $20\ \text{s}$ to reach Earth.

1. a) Express this time in seconds.
2. b) Calculate the distance between the Sun and Earth in kilometres, using $c = 3\cdot10^{5}\ \text{km/s}$, and express it in standard form.

A light year is the distance light travels in one year: about $9,5\cdot10^{12}\ \text{km}$. Proxima Centauri, the closest star to the Sun, is $4,2$ light years away.

1. a) Calculate this distance in kilometres, in standard form.
2. b) How long does it take for light from Proxima Centauri to reach us?

A torch beam strikes a bathroom mirror making an angle of $30^\circ$ with the normal, the line perpendicular to the mirror. The figure shows the incident ray on the mirror.

1. a) At what angle is the ray reflected, measured from the normal?
2. b) State the law of reflection in your own words.
3. c) Draw the mirror, normal, incident ray and reflected ray in your notebook, and mark the angles.

You can do it on paper and send a photo with your answer.

A light ray strikes a plane mirror making an angle of $40^\circ$ with the mirror surface. The figure shows the incident ray on the mirror.

1. a) What angle does the incident ray make with the normal?
2. b) What is the angle of reflection, measured from the normal?
3. c) Draw the situation in your notebook and label all the angles.

You can do it on paper and send a photo with your answer.

Light travels through water at about $225\,000\ \text{km/s}$, slower than in air, where it travels at about $300\,000\ \text{km/s}$. The figure shows the change in direction of the ray as it passes from air to water.

1. a) Calculate how many times faster light is in air than in water, to one decimal place.
2. b) On passing from air to water, the light ray bends towards the normal: relate this change of direction to the change in speed and name the phenomenon.
3. c) Using this, explain why a straw in a glass of water appears bent.

Let us calculate how many kilometres a light year is.

1. a) How many seconds are in a year of $365$ days? Express your answer in standard form.
2. b) Multiply this time by the speed of light ($c = 3\cdot10^{8}\ \text{m/s}$) and give the distance in metres and in kilometres.
3. c) The star Vega is about $25$ light years away: calculate this distance in kilometres.

Betelgeuse, a star in the constellation of Orion, is about $640$ light years away and astronomers expect it to explode as a supernova one day.

1. a) If telescopes were to observe the explosion tomorrow, when did it actually occur? What was happening, approximately, in Earth's history at that time?
2. b) If the explosion actually occurred today, when would the news reach Earth?
3. c) Explain why astronomers compare observing very distant objects to receiving a letter written a long time ago.