# Physics - Black-Body Curves

> Source: https://ollybritton.com/notes/a-level/physics/topics/black-body-diagrams/ · Updated: 2024-08-25 · Tags: school, physics, astrophysics, year-2

## Flashcards
### 2022-01-26
##### What simplifying model do we use when talking about emitters and absorbers of electromagnetic radiation??
Black bodies.

##### What two main assumptions are made about a black body??
1. They are perfect emitters and absorbers of electromagnetic radiation
2. They emit radiation at all wavelengths

##### Why is the sun yellow if it can be modelled as a near-perfect black body??
Because it is hotter than its surroundings and so is glowing.

##### What is special about how a black body emits radiation at all wavelengths??
It follows a standard curve.

##### What is on the $x$-axis for a black body curve??
Wavelength.

##### What is on the $y$-axis for a black body curve??
Intensity.

##### What does the area under a black body curve represent??
Luminosity.

##### Why do toasters glow red hot??
Because they are like a black body emitting E.M. radiation.

##### ![PHOTO BLACK BODY CURVE](paste-832607434db02ad6c2c916b90f22949315702087.jpg) What is this??
A black body curve.

##### ![PHOTO BLACK BODY CURVE](paste-832607434db02ad6c2c916b90f22949315702087.jpg) What does a black body curve show you??
The distribution of wavelengths emitted by a black body.

##### ![PHOTO BLACK BODY CURVE](paste-832607434db02ad6c2c916b90f22949315702087.jpg) What happens to the position of the peak for a cooler object??
It will have a lower peak and move to the right.

![PHOTO BLACK BODY CURVE MANY](paste-8662729375ff4eda60b88e8090dd9be3da27407d.jpg)

##### ![PHOTO BLACK BODY CURVE](paste-832607434db02ad6c2c916b90f22949315702087.jpg) What happens to the position of the peak for a hotter object??
It will have a higher peak and move to the left.

![PHOTO BLACK BODY CURVE MANY](paste-8662729375ff4eda60b88e8090dd9be3da27407d.jpg)

##### What does Wien's displacement law state??
$$
\lambda_\text{max} \propto \frac{1}{T}
$$

##### What law says that the peak wavelength emitted by an object is proportional to one over its temperature??
Wien's displacement law.

##### What is the constant of proportionality in Wien's displacement law, $\lambda_\text{max} \propto \frac{1}{T}$??
Wien's constant.

##### Why is Wien's displacement law, $\lambda_\text{max} \propto \frac{1}{T}$, useful??
Because it lets you work out the temperature of a star by measuring only its wavelength.

##### What does Stefan's law state??
The total power radiated per unit surface area is directly proportional to the fourth power of the absolute temperature of black body.

##### What is Stefan's law in equation form for $L$ in terms of radius, temperature and Stefan's constant??
$$
L = 4\pi r^2 \sigma T^4
$$

##### What is Stefan's law in terms of proportionality??
$$
L \propto T^4
$$

##### What law says that $L \propto T^4$??
Stefan's law.

##### By combining Stefan's law and Wien's displacement law, what can you work out from just looking at starlight??
The radius of a star.

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