Hershel experiment regarding IR radiation

In summary, the Hershel experiment showed that red light has a higher temperature than violet light, even though violet light has a higher energy. This is because sunlight has a stronger infrared component, and the energy per photon is larger for ultraviolet light. The intensity of the light also plays a role in determining the temperature of the thermometer, and a quantitative analysis would need to consider the wavelength-dependency of the refractive index and the fraction of light that gets absorbed. The main takeaway from the experiment was the discovery of infrared light, which has a heating effect even when not visible.
  • #1
mildpiranha
8
0
I have been struggling with the following; (apologies if this is a simple question)

After reading the Hershel experiment where with a prism light was used to split white light into a spectrum of colours and the red light was warmer than the violet light, then he placed a detector past the red and discovered infrared where his detector registered a higher temperature.

My question is why does the red light have a higher temperature than the violet, when the violet light has a higher energy?

Also if he placed a detector past the violet section in the ultraviolet region would he have detected a colder temperature?

Thank you for your help
 
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  • #2
Light does not have a temperature.

The temperature of the thermometer depends on the light intensity (and the absorption coefficient of the surface). Sunlight has a strong infrared component, the intensity in the ultraviolet range is low (and below a certain wavelength, the prism will just absorb the light).

The energy per photon is larger for ultraviolet light.
 
  • #3
Thank you for your response.

To clarify does that mean that in the case of this experiment that the red light increased the temperature of the thermometer more than the violet light did as its intensity was greater?

So, if I had equal intensities of red light on one thermometer and violet light on another thermometer would the thermometer which had the violet light on have a greater intensity due to the greater energy of violet light?
 
  • #4
mildpiranha said:
To clarify does that mean that in the case of this experiment that the red light increased the temperature of the thermometer more than the violet light did as its intensity was greater?
Right.

So, if I had equal intensities of red light on one thermometer and violet light on another thermometer would the thermometer which had the violet light on have a greater intensity due to the greater energy of violet light?
Not in general. Intensity already accounts for this difference, its unit is W/m^2 (power per area). The area of the thermometer stays the same, and the absorbed power determines the temperature reading.

A quantative analysis has to be more careful: The thermometer will always receive light in some wavelength range, where the size of this range depends on the wavelength-dependency of the refractive index of the prism, and the received power depends on this size. Also, not the total intensity hitting the thermometer will get absorbed, and this fraction can depend on the wavelength. But those are technical details - the interesting result of the experiment was the notable heating effect where no light was visible.
 
  • #5


I can provide an explanation for the results of Hershel's experiment. The key factor here is the concept of energy distribution. While violet light does have a higher energy per photon compared to red light, the number of photons emitted by each color is not the same. In fact, there are significantly more red photons than violet photons in white light. This means that, even though each red photon has less energy, the overall energy distribution of red light is greater than that of violet light. This is why the red light has a higher temperature than the violet light.

In regards to your second question, it is possible that a detector placed past the violet section in the ultraviolet region would detect a colder temperature. This is because the energy distribution of ultraviolet light is even lower than that of violet light. However, it is important to note that the concept of temperature becomes less relevant when dealing with high energy radiation such as ultraviolet or X-rays. In these cases, it is more accurate to measure the intensity or flux of the radiation instead of temperature.

I hope this explanation helps in understanding Hershel's experiment and the concept of energy distribution in light. Keep asking questions and exploring the world around us!
 

1. What is the Hershel experiment regarding IR radiation?

The Hershel experiment refers to an experiment conducted by William Hershel in the 19th century to investigate the properties of infrared (IR) radiation. He used a prism to split sunlight into different colors and measured the temperature of each color with a thermometer. He noticed that the temperature was highest beyond the visible red light, which led him to discover the existence of IR radiation.

2. How did Hershel conduct his experiment?

Hershel conducted his experiment by using a prism to split sunlight into different colors and measuring the temperature of each color with a thermometer. He also used a control experiment where he placed the thermometer beyond the red light to measure the temperature of the invisible radiation.

3. What were the results of Hershel's experiment?

The results of Hershel's experiment showed that there was an increase in temperature beyond the visible red light, indicating the presence of invisible radiation - which he named "heat rays" or infrared radiation. This discovery was significant as it provided evidence that there are forms of energy beyond what is visible to the human eye.

4. How did Hershel's experiment contribute to our understanding of IR radiation?

Hershel's experiment was groundbreaking as it provided evidence for the existence of IR radiation and its properties. It also helped us understand that there are forms of energy beyond what is visible to the human eye and that these forms of energy have an impact on temperature.

5. What are the practical applications of Hershel's experiment?

Hershel's experiment has practical applications in various fields, including astronomy, meteorology, and technology. In astronomy, IR radiation is used to study celestial objects, while in meteorology, it helps us understand weather patterns. In technology, IR radiation is utilized in devices such as night vision cameras and remote controls.

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