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# Classifying stars for SQA Higher Physics

1. Black bodies
2. Inverse square law

A black body radiator is a perfect emitter and absorber of all wavelengths of radiation. Stars can be approximated as black bodies. It is called a black body because it would appear black when any wavelength of light is shone on it, since it would absorb all of it. The radiation emitted by black bodies is over a continuous range of wavelengths with a peak wavelength that has a higher maximum intensity. The peak wavelength of a black body is determined by its temperature. The hotter the black body, the shorter the peak wavelength of radiation will be.

The inverse square law states that the intensity of a star is inversely propotional to the distance from the observer to the star. This is given by the following formula: I = P/4ฯdยฒ. The denominator of this formula is the expression for the surface area of a sphere with radius d. This is because the light emitted by a star is approximated to be equal in all directions from the source point, thus giving the shape of the sphere.

# 1

Describe the radiation emitted by black bodies.

The radiation emitted by black bodies is over a continuous range of wavelengths with a peak wavelength that has a higher maximum intensity.

# 2

What is the inverse square law?

The inverse square law states that the intensity of a star is inversely propotional to the distance from the observer to the star.

# 3

Explain the name of a black body.

It is called a black body because it would appear black when any wavelength of light is shone on it, since it would absorb all of it.

# 4

A star is at a distance of 2 ร 10ยนโท m from Earth. Given that the power output of the star is 5 ร 10ยฒโถ W, calculate the intensity of the star.

I = (5 ร 10ยฒโถ W)/4ฯ(2 ร 10ยนโท m)ยฒ = 9.95 ร 10โปยนโฐ Wmโปยฒ (to 3 significant figures).

# 5

The black body curves for two objects are given. State which one is at a higher temperature.

B is at a higher temperature than A.

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