# +2 Physics - Unit 5 - Electromagnetic Waves And Wave Optics

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Prepared by P. ILAIYARAJA M. Sc.,B. Ed., PG ASST IN PHYSICS , GHSS, PANCHANATHIKKULAM EAST, NAGAI DT.
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• 1.

### In an electromagnetic wave

• A.

Power is transmitted in a direction perpendicular to both the fields

• B.

Power is transmitted along electric field

• C.

Power is transmitted along magnetic field

• D.

power is equally transferred along the electric and magnetic fields

A. Power is transmitted in a direction perpendicular to both the fields
Explanation
In an electromagnetic wave, power is transmitted in a direction perpendicular to both the electric and magnetic fields. This is because electromagnetic waves consist of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of wave propagation. The energy carried by the wave is transferred in a direction perpendicular to these fields, allowing the wave to propagate through space.

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• 2.

### Electromagnetic waves are

• A.

Transverse

• B.

Longitudinal

• C.

May be longitudinal or transverse

• D.

Neither longitudinal nor transverse

A. Transverse
Explanation
Electromagnetic waves are transverse because they consist of oscillating electric and magnetic fields that are perpendicular to the direction of wave propagation. This means that the particles of the medium through which the wave is traveling move perpendicular to the direction of the wave. Transverse waves can be observed in various forms such as visible light, radio waves, and X-rays.

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• 3.

### Refractive index of glass is 1.5. Time taken for light to pass through a glass plate of thickness 10 cm is

• A.

(4)

• B.

(3)

• C.

(2)

• D.

(1)

A. (4)
Explanation
The refractive index of a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in that medium. In this case, the refractive index of glass is given as 1.5. The time taken for light to pass through a medium is directly proportional to the thickness of the medium and inversely proportional to the speed of light in that medium. Therefore, if the refractive index is higher, it means that the speed of light in the medium is lower. Since the thickness of the glass plate is given as 10 cm, the time taken for light to pass through it will be longer compared to a medium with a lower refractive index. Hence, the correct answer is (4).

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• 4.

### In an electromagnetic wave the phase difference between electric field and magnetic field is

• A.

Zero

• B.

π/2

• C.

π/4

• D.

π

A. Zero
Explanation
In an electromagnetic wave, the electric and magnetic fields are perpendicular to each other and oscillate in sync. This means that the peaks and troughs of the electric field correspond to the peaks and troughs of the magnetic field. As a result, there is no phase difference between the two fields, and they are said to be in phase. Therefore, the correct answer is zero.

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• 5.

### Atomic spectrum should be

• A.

Pure line spectrum

• B.

Absorption line spectrum

• C.

Absorption band spectrum.

• D.

Emission band spectrum

A. Pure line spectrum
Explanation
A pure line spectrum refers to a spectrum consisting of discrete lines of different wavelengths, with no other wavelengths present. This indicates that the atoms or molecules in the sample are only emitting or absorbing light at specific wavelengths, corresponding to specific energy transitions. This is typically observed in gases or isolated atoms, where the energy levels are well defined. In contrast, an absorption line spectrum would show dark lines against a continuous spectrum, indicating that certain wavelengths of light are being absorbed by the sample. Absorption band spectrum and emission band spectrum refer to spectra with a range of wavelengths, indicating that multiple energy transitions are occurring simultaneously.

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• 6.

### When a drop of water is introduced between the glass plate and plano convex lens in Newton’s rings system, the ring system

• A.

Contracts

• B.

Expands

• C.

First expands, then contracts

• D.

Remains same

A. Contracts
Explanation
When a drop of water is introduced between the glass plate and plano convex lens in Newton's rings system, the ring system contracts. This is because the presence of the water drop causes a change in the refractive index between the glass plate and the lens. This change in refractive index leads to a decrease in the path difference between the reflected and transmitted light waves, causing the rings to contract.

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• 7.

### A beam of monochromatic light enters from vacuum into a medium of refractive index μ. The ratio of the wavelengths of the incident and refracted waves is

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

A. (1)
Explanation
The ratio of the wavelengths of the incident and refracted waves is given by the equation λ1/λ2 = v1/v2, where λ1 and λ2 are the wavelengths, and v1 and v2 are the velocities of light in the two mediums. In this case, the incident light is in vacuum, so its velocity is c, the speed of light in vacuum. The refracted light is in a medium with refractive index μ, so its velocity is c/μ. Substituting these values into the equation, we get λ1/λ2 = c/(c/μ) = μ. Therefore, the ratio of the wavelengths is equal to the refractive index of the medium, which is option (1).

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• 8.

### If the wavelength of the light is reduced to one fourth, then the amount of scattering is

• A.

Increased by 256 times

• B.

Decreased by 256 times decreased by 256 times

• C.

increased by 16 times

• D.

Decreased by 16 times

A. Increased by 256 times
Explanation
When the wavelength of light is reduced to one fourth, the amount of scattering is increased by 256 times. This is because the amount of scattering is inversely proportional to the fourth power of the wavelength. When the wavelength is reduced to one fourth, the denominator in the inverse proportion increases by a factor of 4^4, which is equal to 256. Therefore, the amount of scattering is increased by 256 times.

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• 9.

• A.

10

• B.

8

• C.

4

• D.

2

A. 10
• 10.

• A.

π

• B.

• C.

3π/2

• D.

π/2

A. π
• 11.

### In Young’s experiment, the third bright band for wavelength of light 6000 Å coincides with the fourth bright band for another source in the same arrangement. The wave length of the another source is

• A.

4500 Å

• B.

6000 Å

• C.

5000 Å

• D.

4000 Å

A. 4500 Å
Explanation
In Young's experiment, the bright bands are formed due to constructive interference of light waves. The position of the bright bands depends on the wavelength of the light. In this case, the third bright band for a wavelength of 6000 Å coincides with the fourth bright band for another source. This means that the path difference between the two sources is the same for these bands. Since the path difference is directly related to the wavelength of light, the wavelength of the other source must be smaller, which is 4500 Å.

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• 12.

### A light of wavelength 6000 Å is incident normally on a grating 0.005 m wide with 2500 lines. Then the maximum order is

• A.

3

• B.

2

• C.

1

• D.

4

A. 3
Explanation
The maximum order of a diffraction grating can be calculated by using the formula:
mλ = d sinθ
where m is the order, λ is the wavelength, d is the distance between the lines on the grating, and θ is the angle of diffraction. In this case, the light is incident normally on the grating, so the angle of diffraction is 0 degrees. Plugging in the values given in the question, we get:
m * 6000 = 0.005 * sin(0)
Since the sine of 0 is 0, the equation simplifies to:
m * 6000 = 0
This means that m can be any value, including 3. Therefore, the maximum order is 3.

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• 13.

### A diffraction pattern is obtained using a beam of red light. What happens if the red light is replaced by blue light?

• A.

Diffraction pattern becomes narrower and crowded together

• B.

Bands disappear

• C.

no change

• D.

Diffraction pattern becomes broader and farther apart

A. Diffraction pattern becomes narrower and crowded together
Explanation
When red light is replaced by blue light in a diffraction pattern, the pattern becomes narrower and crowded together. This is because blue light has a shorter wavelength compared to red light. Diffraction is the bending of light waves around obstacles or through narrow openings. The shorter wavelength of blue light causes it to diffract more, resulting in a narrower pattern with closely spaced bands.

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• 14.

### The refractive index of the medium, for the polarising angle

• A.

1.732

• B.

1.414

• C.

1.5

• D.

1.468

A. 1.732
Explanation
The refractive index of a medium determines how much light bends when it passes through that medium. The polarising angle refers to the angle at which light is polarized when it passes from one medium to another. In this case, the correct answer of 1.732 suggests that the refractive index of the medium at the polarising angle is 1.732. This means that light passing through this medium at the polarising angle will bend or refract at a specific angle determined by this refractive index.

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