A source of light is placed at a distance of 50 cm from a photocell and the stopping potential is found to be V0. If the distance between the lightsource and photocell is made 25 cm, the newstopping potential will be [NEET Kar. 2013]a)2V0b)V0/2c)V0d)4V0Correct answer is option 'C'. Can you explain this answer?

Question

Answer

By changing the position of source of light from photocell, there will be a change in the intensity of light falling on photocell.
As stopping potential is independent of the intensity of the incident light, hence stopping potential remains same i.e., V0.

Answer

Explanation:

When a source of light is placed at a distance from a photocell, the intensity of light reaching the photocell depends on the inverse square of the distance. In this case, when the distance between the light source and photocell is halved, the intensity of light reaching the photocell will increase by a factor of 4 (since (1/2)^2 = 1/4).

Stopping Potential and Intensity of Light:

The stopping potential is the minimum potential difference that must be applied across a photocell to stop the emission of photoelectrons. It depends on the intensity of the incident light.

As the intensity of light increases, the number of photons incident on the photocell per unit time increases. Therefore, the number of photoelectrons emitted per unit time also increases. This leads to an increase in the photocurrent.

Relation between Stopping Potential and Intensity:

According to the photoelectric equation, the maximum kinetic energy of emitted photoelectrons is given by:

K.E. = hf - φ

Where:
- K.E. is the maximum kinetic energy of photoelectrons
- h is the Planck's constant
- f is the frequency of incident light
- φ is the work function of the material

The stopping potential (V0) is the potential difference required to stop the emission of photoelectrons, which is equal to the maximum kinetic energy of the photoelectrons.

Therefore, we can write:

V0 = hf - φ

From this equation, we can see that the stopping potential (V0) depends on the frequency of incident light (f).

Effect of Halving the Distance:

When the distance between the light source and photocell is halved, the intensity of light reaching the photocell increases by a factor of 4. This increase in intensity will not change the frequency of the incident light.

Therefore, if the stopping potential is V0 initially, it will remain the same when the distance is halved.

Conclusion:

Therefore, when the distance between the light source and photocell is halved, the stopping potential remains the same. The correct answer to the given question is option 'C' - V0.

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