Green light of mercury has a wavelength of 5 .5 × 10 − 7 m in air. What is its frequency in MHz and period in s ?

Given wavelength of light λ = 5 . 5 × 10 - 7 m Assume velocity of light in air c = 3 × 10 8 m / s As the velocity is given by c = ν . λ where ν =frequency Putting the values in the formula, we have 3 × 10 8 = 5 . 5 × 10 - 7 × ν or ν = 3 . 0 × 10 8 5 . 5 × 10 - 7 = 5 . 45 × 10 14 Hz As time period = 1 frequenc y = 1 5 . 45 × 10 14 = 1 . 83 × 10 - 15 or 1 . 83 × 10 - 9 μs .

Derive the law of reflection using Huygen ' s Wave Theory.

According to Huygen ' s principle, each point of the wavefront is the source of a secondary disturbance and the wavelets emanating from these points spread out in all directions with the speed of the wave and if we draw a common tangent to all these spheres, we obtain the new position of the wavefront at a later time. Let XY be a plane reflecting surface and AMB the incident plane wavefront. All the particles on AB will be in vibrating in phase. Let i be the angle of incidence and r the angle of reflection. In the time t , the disturbance at A reaches C , the secondary waves from the point B must travel a distance BD equal to AC . With the point B as centre and radius equal to AC a sphere is constructed. From the point C , we draw tangents CD and CD ' . Then, BD = BD ' . In ∆ BAC and ∆ BDC , BC is common, BD = AC , and ∠ BAC = ∠ BDC = 90 ° ∴ ∆ B A C ≅ ∆ B D C and ∠ ABC = ∠ BCD But ∠ ABC = i and ∠ BCD = r , i.e., i = r or the angle of incidence is equal to angle of reflection. Since AB , AD and XY are in the same plane. Therefore, the incident ray, reflected ray and normal at the point of incidence are all in the same plane. This is first law of reflection.

E M B I B E

Physics Crash Course (Based on Revised Syllabus-2023)>Chapter 6 - Wave Nature of Light : Huygens' Principle>Exercise>Q 20

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12th ICSE

IMPORTANT

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Explain what is optical path length. How is it different from actual path length?

Important Questions on Wave Nature of Light : Huygens' Principle

MEDIUM

12th ICSE

IMPORTANT

Physics Crash Course (Based on Revised Syllabus-2023)>Chapter 6 - Wave Nature of Light : Huygens' Principle>Exercise>Q 21

Define optical path and prove that optical path $(Δ) = nd$ , where $n$ is the refractive index of medium other than the air and ‘ $d$ ’ is the distance travelled in the medium. If a mica sheet of thickness ‘ $t$ ’ and refractive index ‘ $n$ ’ is placed in the path of a ray of light, by how much the optical path is included?

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12th ICSE

IMPORTANT

Physics Crash Course (Based on Revised Syllabus-2023)>Chapter 6 - Wave Nature of Light : Huygens' Principle>Exercise>Q 22

Green light of mercury has a wavelength of

5 .5 \times 10^{- 7} m

in air. What is its frequency in

MHz

and period in

s

?

MEDIUM

12th ICSE

IMPORTANT

Physics Crash Course (Based on Revised Syllabus-2023)>Chapter 6 - Wave Nature of Light : Huygens' Principle>Exercise>Q 23

A high-powered laser beam $(λ = 600 nm)$ with a beam diameter of $10 cm$ is aimed at the Moon, $3.8 \times 10^{5} km$ distant. The beam spreads only because of diffraction. The angular location of the edge of the central diffraction disk is given by