12th Grade Physics Exam: Quiz!

1. The electrons ejected from metals due to effect of light are called

Primary electrons

Thermions

Photoelectrons

When light shines on a metal surface, it can cause the electrons in the metal to be ejected. These ejected electrons are called photoelectrons. This phenomenon is known as the photoelectric effect. The energy of the incident light is transferred to the electrons, giving them enough energy to overcome the attractive forces within the metal and escape. These photoelectrons can be detected and measured, providing valuable information about the properties of the metal and the characteristics of the incident light.

Explanation

When light shines on a metal surface, it can cause the electrons in the metal to be ejected. These ejected electrons are called photoelectrons. This phenomenon is known as the photoelectric effect. The energy of the incident light is transferred to the electrons, giving them enough energy to overcome the attractive forces within the metal and escape. These photoelectrons can be detected and measured, providing valuable information about the properties of the metal and the characteristics of the incident light.

2. According to Huygens' principle, light is a form of

Particle

Wave

Radiation

According to Huygens' principle, light is considered to be a wave. This principle states that every point on a wavefront acts as a source of secondary spherical waves, and the sum of these waves determines the new position of the wavefront. This concept is used to explain various optical phenomena, such as diffraction and interference, which are characteristic of wave behavior. Therefore, the correct answer is wave.

Explanation

According to Huygens' principle, light is considered to be a wave. This principle states that every point on a wavefront acts as a source of secondary spherical waves, and the sum of these waves determines the new position of the wavefront. This concept is used to explain various optical phenomena, such as diffraction and interference, which are characteristic of wave behavior. Therefore, the correct answer is wave.

3. The wavefront due to a source situated at infinity is:

Spherical

Planar

Cylindrical

When a source is situated at infinity, the wavefront it produces appears as a planar wavefront. This means that the wavefront is flat and extends infinitely in all directions. A planar wavefront is characterized by parallel wavefront lines, which indicates that the wave is propagating in a straight line. In contrast, a spherical wavefront would indicate that the wave is originating from a point source, and a cylindrical wavefront would suggest that the wave is spreading out in a cylindrical shape. Therefore, the correct answer is planar.

Explanation

When a source is situated at infinity, the wavefront it produces appears as a planar wavefront. This means that the wavefront is flat and extends infinitely in all directions. A planar wavefront is characterized by parallel wavefront lines, which indicates that the wave is propagating in a straight line. In contrast, a spherical wavefront would indicate that the wave is originating from a point source, and a cylindrical wavefront would suggest that the wave is spreading out in a cylindrical shape. Therefore, the correct answer is planar.

4. A laser beam is coherent because it contains:

(a) waves of several wavelengths.

(b) coherent waves of a single wavelength.

(c) coherent waves of several wavelengths

A laser beam is coherent because it contains coherent waves of a single wavelength. Coherence refers to the property of waves having a constant phase relationship with each other. In a laser, the light waves are produced through stimulated emission, where photons are emitted in phase with each other. This results in a beam of light with a single wavelength and a constant phase relationship between the waves. This coherence allows the laser beam to have a narrow, focused beam and to exhibit interference effects such as diffraction.

Explanation

A laser beam is coherent because it contains coherent waves of a single wavelength. Coherence refers to the property of waves having a constant phase relationship with each other. In a laser, the light waves are produced through stimulated emission, where photons are emitted in phase with each other. This results in a beam of light with a single wavelength and a constant phase relationship between the waves. This coherence allows the laser beam to have a narrow, focused beam and to exhibit interference effects such as diffraction.

5. An electron, an alpha-particle, and a proton have the same kinetic energy. Which of these particles has the shortest de Broglie wavelength?

A. electron

B. proton

C. alpha-particle

de Broglie wavelength is given by λ =h/ p -(1) Also KE is given by K = p^ 2 / 2m --(2) From (1) and (2) we can say that For the same kinetic energy K, the de Broglie wavelength associated with the particle is inversely proportional to the square root of their masses. Now alpha-particle is heaviest among them Hence, alpha-particle has the shortest de Broglie wavelength. So (c) is correct

Explanation

de Broglie wavelength is given by λ =h/ p -(1) Also KE is given by K = p^ 2 / 2m --(2) From (1) and (2) we can say that For the same kinetic energy K, the de Broglie wavelength associated with the particle is inversely proportional to the square root of their masses. Now alpha-particle is heaviest among them Hence, alpha-particle has the shortest de Broglie wavelength. So (c) is correct

6. In an experiment to find the focal length of a concave mirror, a graph is drawn between the magnitude of u and v. The graph looks like?

Option 1

Option 2

Option 3

u – v curve is a rectangular parabola.

Explanation

u – v curve is a rectangular parabola.

7. How many electron volts makeup 1 Joule?

(a) 6.25×10^18 eV.

(b) 1.25×10^181 eV

(c) 6.25×10^16 eV.

1 eV= 1.6 × 10 ^−19 J
So 1 J= 6.25×10 ^18 eV
Hence (a) is the correct option

Explanation

1 eV= 1.6 × 10 ^−19 J
So 1 J= 6.25×10 ^18 eV
Hence (a) is the correct option

8. A converging lens is used to form an image on a screen. When the upper half of the lens is covered by an opaque screen.

(a) half the image will disappear.

(b) intensity of image will decrease but complete image is formed.

(c) intensity of image will increase but image is not distinct.

because focal length of lens does not change but amount of light passing through lens becomes half.

Explanation

because focal length of lens does not change but amount of light passing through lens becomes half.

9. In optical fibers, the refractive index of the core is:

Greater than that of the cladding.

Smaller than that of the cladding.

Equal to that of the cladding.

R.I. of core is greater than that of the cladding for total internal reflection to occur.

Explanation

R.I. of core is greater than that of the cladding for total internal reflection to occur.

10. The refractive index of the material of an equilateral prism is √3. What is the angle of minimum deviation?

(a) 45°

(b) 60°

(c) 37°

Option 4

use Prism formula for minimum deviation condition

Explanation

use Prism formula for minimum deviation condition

11. A metal coin is at bottom of a beaker filled with a liquid of refractive index = 4/3 to a height of 6 cm. To an observer looking from above the surface of the liquid, coin will appear at a depth.

(a) 1.5 cm

(b) 4.5 cm

(c) 6.75 cm

Option 4

R.I = Real depth / apparent depth

Explanation

R.I = Real depth / apparent depth

12. If the maximum Kinetic energy of electrons emitted in a photo-cell is 6 eV. What is the stopping potential?

3 V

12 V

6 V

The stopping potential in a photo-cell is the minimum potential difference required to stop the flow of electrons. In this case, the maximum kinetic energy of the emitted electrons is given as 6 eV. The stopping potential is equal to the maximum kinetic energy, which means it is also 6 V.

Explanation

The stopping potential in a photo-cell is the minimum potential difference required to stop the flow of electrons. In this case, the maximum kinetic energy of the emitted electrons is given as 6 eV. The stopping potential is equal to the maximum kinetic energy, which means it is also 6 V.

13. The frequency of the incident light falling on a photosensitive metal plate is doubled, stopping potential will be:

Double the earlier value

Unchanged

More than doubled

ⅇV0=hν-∅0 use this formula. ⅇV0 + ∅0 =hν

Explanation

ⅇV0=hν-∅0 use this formula. ⅇV0 + ∅0 =hν

14. Two coherent monochromatic light beams of intensities I and 41 superimpose. The maximum and minimum possible intensities in the resulting beam are:

(a) 5I and I

(b) 5I and 3I

(d) 9I and I

I max= I1 + I2 + 2 √ (I1 X I2)= 9 I
I min = I1 + I2 - 2 √ (I1+I2) = 1 I

Explanation

I max= I1 + I2 + 2 √ (I1 X I2)= 9 I
I min = I1 + I2 - 2 √ (I1+I2) = 1 I