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SUBJECT VIIIь. LIGHT.

Advanced Stage and Honours.

1899.

EXAMINERS:

PROFESSOR A. W. RÜCKER, M.A., SEC.R.S.,

PROFESSOR W. GRYLLS ADAMS, M.A., D.Sc., F.R.S.,

AND

PRINCIPAL R. T. GLAZEBROOK, M.A., F.R.S.

GENERAL INSTRUCTIONS.

If the rules are not attended to, your paper will be cancelled.

You may take the Advanced Stage, or Part I. of Honours, or (if eligible) Part II. of Honours, but you must confine yourself to one of them.

The marks allotted to each question are given in brackets.
Put the number of the question before your answer.

You are to confine your answers strictly to the questions proposed.

Your name is not given to the Examiners, and you are forbidden to write to them about your answers.

The examination in this subject lasts for three hours.

Advanced Stage.

INSTRUCTIONS.

Read the General Instructions above.

You are permitted to answer only seven questions.

31. Light from a small luminous object traverses a convex lens directly, and after reflection at a plane mirror again passes through the lens and forms an image coincident with the object.

Show that the image formed may be either erect or inverted, and trace the paths of the rays in the two cases.

32. Explain (without proving the necessary formula) how to use a goniometer to determine the angle and the refractive index of a glass prism.

(28.)

33. Draw and explain a figure showing the oblique refraction of a beam of light by a spherical surface, and illustrating the formation of primary and secondary foci. (28.) 34. Describe a method of determining the magnifying power of a telescope. (28.) 35. Show how the refraction of light by a plane surface is accounted for by the wave theory. What is the meaning of the refractive index according to this theory? (28.) 36. Obtain the relation between the critical angle and the refractive index of a substance, and describe carefully the experimental arrangement you would employ to determine the refractive index of glass by observations of the critical angle. (28.) 37. Distinguish between ordinary, plane polarised and circularly polarised light. What tests would you employ to determine if a given beam of light is circularly polarised? (30.) 38. Describe a method of producing Fresnel's interference bands, and show how they may be used to measure the wave length of light.

39. Write a short essay on colour-blindness.

(28.) (30.)

HONOURS-PART I.

INSTRUCTIONS.

Read the General Instructions on page 1.

You are not permitted to answer more than five questions.

61. When an opaque rod, held parallel to the edge of a glass prism, is viewed by an eye applied to the prism in a position of minimum deviation, explain the appearance presented as regards the form and colour of the image of the rod. (60.) 62. Explain how an achromatic image of an object may be obtained by means of a combination of lenses. The refractive indices of two substances are 1.4 and 1.7 and their dispersive powers are as 3.5 to 5, of what focal length should the lenses be in order that the achromatic combination may have a 6-ft. focus?

(60.)

63. Describe some form of colour box and show how to use it to prove that a given colour may be matched by a mixture of three other colours.

(60.) 64. Homogeneous light from a narrow slit falls very obliquely on a plane surface parallel to the slit. Explain the interference bands which are formed and show how to use them to determine the wave length of the light, obtaining the necessary formula. (60.) 65. Explain the properties of a circularly polarised beam of light and how it may be produced; how may it be distinguished from a beam of ordinary light?

(60.)

66. Write a short essay on the measurement and comparison of the refractive indices of gases. (60.)

67. Describe the experiments performed by Dr. Kerr on the reflection of plane polarised light from the surface of a magnetised body.

(60.)

HONOURS-PART II.

PAPER I.

NOTE.-No Candidate is eligible for examination in Part II. of Honours who has not obtained a pass in Honours--Part I. in 1898, or a 1st or 2nd Class in Honours of the same subject in a previous year.

INSTRUCTIONS.

Read the General Instructions on page 1.

You are not permitted to answer more than five questions. Candidates who do well in Paper I. will be summoned to South Kensington to undergo a further examination, which will consist of (1) another paper, and (2) a practical examination in the laboratory.

91. If a candle be placed upon a table at a given distance from a point in the table, what must be the height of the candle that the illumination at that point may be the greatest possible?

(40.)

92. Describe the experiments performed by Dr. Kerr on the reflection of plane polarised light from the surface of a magnetised body.

(40.)

93. Show how it follows from observations on the light scattered by small particles that the direction of vibration of plane polarised light is perpendicular to the plane of polari

sation.

(40.)

94. The disturbance in a plane wave of plane polarised light is

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Verify the result that if S be an element of this wave at a point Q, and P a point in front of this wave at a distance from Q, then the disturbance at P due to

&S is

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Where is the angle between PQ and the wave normal, that between PQ and the direction of vibration at P.

(40.) 95. Describe and explain the action of apparatus for determining the effect on the velocity of light in a gas caused by variations of temperature and of density in the gas. (40.) 96. Investigate the phenomena when a beam of converging plane polarised light passes through a thin crystalline plate and is analysed by a Nicol prism.

(40.) 97. Write a short essay on the intensities of the rays reflected and refracted respectively at the surface of a non-crystalline medium.

(40.)

1899.

HONOURS-PART II.

PAPER II.

INSTRUCTIONS.

You are not permitted to answer more than five questions, of which one must be in Section A.

121. Write an essay on

SECTION A.

Theories of Aberration; or

The evidence for Fresnel's Law of Double Refraction. (40.)

122. Describe and criticise recent measurements of the wave

length of light.

(40.)

SECTION B.

123. Give a critical account of some important modern experimental research in Light (not referred to in any other question).

(40.)

124. Show how it follows from observations on the light scattered by small particles that the direction of vibration is perpendicular to the plane of polarisation. Explain carefully how you define the term Plane of Polarisation.

(40.) 125. What do you understand by anomalous dispersion? Give a general explanation of some theory of the connection. between ether and matter which will account for anomalous dispersion.

(40.) 126. Investigate the relation between the angle of incidence of light polarised in the plane of incidence and the change of phase when total internal reflexion occurs.

(40.) 127. Describe and explain how to produce the phenomena known as Haidinger's brushes, and give a physical explanation of them.

(40.)

Laboratory Work.

HONOURS (PART II.). PRACTICAL EXAMINATION. 1. Adjust the given reflecting grating on the spectrometer, and having given the wave-length of sodium light determine the number of lines per centimetre in the grating.

2. Measure the refractive index of the given liquid for the C and F hydrogen lines.

3. Measure the focal length and the radii of curvature of the surfaces of the given lens, and hence calculate the refractive index of the glass.

4. Measure the polarising angle of the given substance, and hence deduce its refractive index.

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