Black body emission and Energy from the Sun

The wavelength of the CMB is 1.1 mm. What is the absolute temperature associated with this wavelength? Remember to include units with your answer.

• 2.9 K

• 2.6 K

• 2.9 × 10^–3 m K

• 2.9 × 10^–3 mK

Light from a distant star is detected on Earth. The distance between each marked point on the diagram represents one-quarter of the total distance between the star and the Earth.

At which point can the apparent brightness of the star be measured?

• A

• B

• C

• D

The Earth orbits the Sun in an elliptical orbit. The closest distance (perihelion) is 147.1 million km, whilst the furthest distance from Earth to Sun (aphelion) is 152.1 million km.

By what percentage of its smallest value does the apparent brightness of the Sun (otherwise known as the "solar constant") change in the course of a year? Write your answer to 1 significant figure.

• 1 %

• 3 %

• 4 %

• 7 %

Star A and star B have the same apparent brightness when viewed from Earth. However the luminosity of star A is twice the luminosity of star B.

What is the ratio of the distance of star A to the distance of star B as measured from the Earth?

Give your answer to two significant figures.

• 0.50

• 1.0

• 1.4

• 2.0

A planet is subject to incoming stellar radiation at an average intensity of 2.50 kW m⁻² over its whole surface. If it has a constant temperature of 244 ^∘C, what is the emissivity of the planet? 

• 0.617

• 2.50

• 5.67

• 273

What is the total radiative power of a sphere of radius 1.5 cm, and emissivity 0.5, at 60 °C?

• 2.0 W

• 0.99 W

• 0.14 W

• 0.07 W

Light is shone onto a reflective surface of albedo 0.15 and area 1.5 m² as shown.

What is the total power scattered by half of the surface?

• 34 W

• 45 W

• 192 W

• 225 W

The Earth has an approximate albedo rating of 0.3. Three sources of albedo and amounts of incoming solar radiation reflected are shown in the table.

What percentage does reflection from Earth’s surface make of the total albedo of the Earth?

• 94 %

• 67 %

• 20 %

• 16 %