Refractometer

Question 1

When can refraction be observed?

Answer 1

incident ray striking a boundary surface (not perpendicular!) btw 2 different media

→ direction change of transmitted ray = refraction

Question 2

What is the absolute refractive index?

Answer 2

considers only 1 medium, thus ratio of the speed of light in vacuum (c) to that in the medium (c₁)

n₁ = c/c₁

Question 3

What is the relative refractive index?

Answer 3

explains how much the incident light ray is refracted, thus the ratio of the indices of refraction of the 2 media

n₂₁ = n₂/n₁

Question 4

Explain the relationship btw frequency, wavelength and speed of light in vacuum and in the medium.

How do they change?

On what else does the refractive index depend?

Answer 4

1. due to different indices of refraction: change in c
   * *BUT:** f remains constant
2. because c = λ * f: change in λ

⇒ larger index of refraction → smaller wavelength

ALSO: dependend on temperature

Question 5

What does Snell’s law describe?

Answer 5

describes the relationship btw angles of incidence and refraction, when light passes through a boundary between 2 different media

sin α/sin β = c₁/c₂ = n₂/n₁ = n₂₁

Question 6

What is the consequence of Snell’s law of refraction?

How does the refracted ray behave when the angle of the incident ray changes?

Answer 6

from less dense to more dense medium

increasing angle of incidence α → increasing angle of refraction β

⇒ largest angle of incidence (grazing ray) → largest angle of refraction (critical angle)

BUT always: α > β

Question 7

What is the grazing ray?

Answer 7

angle of incidence traveling along the boundary surface of the 2 media (~ 90°)

Question 8

What is the critical angle?

Formula.

Answer 8

largest angle of refraction (β_c) corresponding to the grazing ray

n₂₁ = n₂/n₁ = 1/sin β_c

⇒ dependent on ratio of refractive indices (= relative refractive index)

Question 9

What is dispersion?

Why does it happen?

Answer 9

decomposition of white light due to change in wavelength

→ rays of different colors are refracted at different angles (e.g. on prisms, and lenses)

Question 10

What is an Amici prism?

Answer 10

compound prism that restores white light from the light that was spread out into colors

→ causing smear of the rim of the Snell’s window

Question 11

What is Snell’s window?

Answer 11

when diffuse light is refracted → rays form a light cone in the denser medium of half-aperture angle equal to β_c

_{(REMEMBER: βc depends on relative refractive index of the 2 media)}

Question 12

How can the refractive index of a solution be calculated?

Answer 12

refractive index depends on concentration

n_solution = n₀ + constant * conc.

Question 13

What is a Abbe refractometer used for?

What are its components?

Answer 13

⇒ measurement of the index of refraction (hence concentration) of a transparent liquid

components:

• mirror
• measuring + illuminating prism
• turnable Amici prism with a knob for color comp.
• telescope
• microscope with scale

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Question 14

What are the conditions for a refractometer to work?

Answer 14

• n_{measuring solution} < n_{measuring prism}
• transparent liquid

Question 15

How does an Abbe refractometer work?

Answer 15

1. ​measuring solution put into slit btw 2 prisms
2. illuminating white light projected to illuminating prism by a mirror
3. diffused on rough surface of illuminating prism → grazing rays produced in solution
4. Amici prism adjusted by color compensator knob → sharp shadowline
5. shadowline centered in telescope
6. scale calibrated in microscope→ refractive index can be read