# Important Questions and Answers: Photons

1. What is photon? Is it wave or particle?

Ans: A photon is a electromagnetic energy or quantum (smallest portion) of radiation. Since it consists of energy according to Planck’s theory of radiation, it is taken as particle. On the other hand, being a wave and having definite amount of energy so can be considered as a wave. So, photon shown dual character of both wave and particle.

2. What is photoelectric effect?

Ans: When light or electromagnetic radiation falls on a metal surface, electrons are emitted from it. The process of emission of electrons from a metallic surface by the incidence of light (photons) is known as photoelectric effect. The emitted electrons are called photoelectrons.

3. Explain the terms threshold frequency, threshold wavelength and thrasehold energy.

Ans: The minimum frequency below which photoelectric emission is not possible is known as threshold frequency.

The maximum wavelength of light which can eject photoelectrons from a surface is called threshold wavelength.

The minimum energy required to eject an electron from the surface is called threshold energy. It is also known as work function.

4. Light from the bulbs falls on the wooden table but no photoelectrons are emitted. Why ?

Ans: A substance emits photoelectrons when the energy of the incident photon is greater than the work function of the substance. The energy of light bulb is less than the work function of the wooden table. So, no photoelectrons are emitted from the wooden table.

5. Why are alkali metals suitable for photoelectric emission?

Ans: Alkali metals like sodium, lithium, potassium has only one valence electron. So, photoelectrons can be easily emitted when photon of small energy is passed through it compared to other metals. Even visible light can emit photoelectrons form the alkali metals.  So, alkali metals are suitable for photoelectric emission.

6. Which has more energy, a photon of infrared or one in the ultraviolet?

Ans: The energy of a photon is given by:

$$E = \frac{hc}{\lambda }$$

Where c is the velocity of light and h is the Planck’s constant.

$$\therefore E \propto \frac{1}{\lambda }$$

From above, we know that energy of the photon is inversely proportional to its wavelength. Since, the wavelength of infrared photon is greater than the wavelength of ultraviolet region, energy of ultraviolet region is greater.

7. What is stopping potential?

Ans: The minimum retarding potential at which photoelectric current becomes zero is called stopping potential. It is denoted by Vo. If m is the mass of emitted photoelectron and vmax is the maximum velocity, then:

$$\frac{1}{2}mv_{max}^{2} = \text{eV}_{o}$$

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