Light

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课程笔记

Reflection of Light

A normal is drawn at right angles to the boundary between two media.
Angles are measured between the ray and the normal: angle of incidence (i) and angle of reflection (r).
Law of reflection: angle of incidence = angle of reflection (i = r).
In a plane mirror, the image is virtual, same size, same distance behind the mirror as the object is in front, and laterally inverted.
Ray diagrams show incident and reflected rays with arrows indicating direction.

Reflection of light showing the normal, angle of incidence (i) and angle of reflection (r).

Investigating Reflection

Aim: To investigate reflection by a plane mirror.
Independent variable: angle of incidence; dependent variable: angle of reflection.
Use a ray box, protractor, paper, pencil, ruler, and plane mirror.
Mark incident and reflected ray positions, then measure angles with a protractor.
Repeat for different angles of incidence to verify i = r.

Apparatus for investigating the reflection of light

Refraction of Light

Refraction occurs when light passes between two transparent media of different optical density.
From less dense to more dense (e.g., air to glass): light bends towards the normal.
From more dense to less dense (e.g., glass to air): light bends away from the normal.
If light travels along the normal, it does not bend.
Frequency remains constant; speed and wavelength change.

Refraction of light through different shapes of perspex blocks (semi-circular, rectangular, prism)

$Refraction of light through different shapes of perspex blocks (semi-circular, rectangular, prism)$

Refractive Index

Refractive index n = speed of light in vacuum / speed of light in material.
Also n = sin i / sin r (Snell's law).
n is always > 1 and has no units.
Higher n means more optically dense (e.g., diamond n ≈ 2.4, glass n ≈ 1.5).

Total Internal Reflection

Total internal reflection (TIR) occurs when light travels from a denser to a less dense medium and the angle of incidence exceeds the critical angle (c).
Critical angle: angle of incidence for which angle of refraction = 90°.
Relationship: sin c = 1/n.
Higher n gives smaller critical angle.
Uses: optical fibres (communications, endoscopes), prisms in periscopes, binoculars.

Endoscope using total internal reflection

Ray Diagrams for Lenses

Converging (convex) lens: brings parallel rays to a focus at the principal focus (f).
Diverging (concave) lens: makes parallel rays diverge; virtual focus on same side as incoming light.
Focal length: distance from lens centre to principal focus.
Three key rays: through optical centre (undeviated), parallel to axis (through focus), through focus (emerges parallel).

Ray diagram construction for a converging lens: object beyond 2f forms a real, diminished image

Real & Virtual Images

Real image: formed where rays converge; can be projected on a screen; always inverted.
Virtual image: formed where rays appear to diverge; cannot be projected; always upright.
Converging lens can produce real or virtual images depending on object position.
Diverging lens always produces a virtual, upright, diminished image.

Ray diagram showing an object, incident rays, reflected rays, virtual rays, and the virtual image formed by a mirror.

Correcting Sight & Dispersion

Long-sightedness: image forms behind retina; corrected with converging (convex) lens.
Short-sightedness: image forms in front of retina; corrected with diverging (concave) lens.
Dispersion: white light splits into colours when refracted by a prism; violet bends most, red least.
Visible spectrum: red, orange, yellow, green, blue, indigo, violet (longest to shortest wavelength).
Monochromatic light: single frequency/colour (e.g., laser).

Long-sightedness (hyperopia) in the human eye

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练习题

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1.What is the law of reflection?
Easy
- AAngle of incidence equals angle of reflection
- BAngle of incidence equals angle of refraction
- CAngle of reflection is twice the angle of incidence
- DAngle of incidence is greater than angle of reflection
2.Which type of image cannot be projected onto a screen?
Easy
- AReal image
- BVirtual image
- CInverted image
- DMagnified image
3.When light passes from air into glass, it bends towards the normal. What happens to the speed of light?
Easy
- AIt increases
- BIt decreases
- CIt stays the same
- DIt becomes zero
4.A ray of light enters a glass block of refractive index 1.5 at an angle of incidence of 30°. What is the angle of refraction? (sin 30° = 0.5)
Medium
- A19.5°
- B30°
- C45°
- D9.7°
5.What is the critical angle for a material with refractive index 1.5?
Medium
- A41.8°
- B25°
- C90°
- D30°
6.Which of the following is a condition for total internal reflection to occur?
Easy
- ALight travels from a less dense to a more dense medium
- BThe angle of incidence is less than the critical angle
- CLight travels from a more dense to a less dense medium and the angle of incidence is greater than the critical angle
- DThe angle of incidence equals the angle of refraction
7.A converging lens forms a real image of an object placed at 2F. Where is the image formed?
Easy
- AAt F
- BAt 2F on the opposite side
- CBeyond 2F
- DBetween F and 2F
8.A student measures the angle of incidence as 40° and the angle of refraction as 25°. What is the refractive index of the medium? (sin 40° = 0.643, sin 25° = 0.423)
Medium
- A1.52
- B0.66
- C1.0
- D2.0

更多题目

9.Which colour of light is refracted the most when white light passes through a prism?
Easy
- ARed
- BGreen
- CBlue
- DViolet
10.An object is placed 10 cm from a converging lens of focal length 6 cm. What is the image distance? (Use 1/f = 1/u + 1/v)
Medium
- A15 cm
- B3.75 cm
- C10 cm
- D6 cm
11.In a plane mirror, the image is always:
Easy
- AReal and inverted
- BVirtual and upright
- CReal and upright
- DVirtual and inverted
12.Which lens is used to correct short-sightedness?
Easy
- AConverging lens
- BDiverging lens
- CConvex lens
- DPlano-convex lens
13.In the same experiment, the object height is 1.5 cm and the image height is 2.4 cm. What is the magnification M?
Easy
- A0.63
- B1.6
- C3.6
- D0.9
14.A student draws a normal line and an incident ray at 30° to the normal for a plane mirror. After tracing the reflected ray, the angle of reflection should be:
Easy
- A30°
- B60°
- C90°
- D15°
15.A student uses a converging lens to form a real image on a screen. She adjusts the screen until the image is sharp. Which technique ensures the sharpest image?
Medium
- AMove the screen slowly back and forth until the image is as clear as possible
- BPlace the screen at a fixed distance and move the lens
- CUse a dimmer light source
- DClose one eye while viewing the image
16.In a ray-tracing experiment to measure refractive index, which is the most sensible precaution to obtain reliable results?
Medium
- AView the bases of the pins
- BCarry out the experiment in a darkened room
- CUse pins that are taller than the height of the block
- DView the pins with one eye closed
17.A student measures the image distance in a plane mirror experiment and obtains a = 4.5 cm and b = 4.5 cm. What does this indicate?
Easy
- AThe image is at the same distance behind the mirror as the object is in front
- BThe image is closer to the mirror than the object
- CThe mirror is not plane
- DThe angles were not measured correctly
18.In a refraction experiment using a transparent block, a student measures a = 3.2 cm and b = 6.9 cm for an angle of incidence of 45°. The refractive index is given by n = 0.71 × (b/a). What is the refractive index?
Medium
- A1.53
- B0.65
- C1.00
- D2.16
19.In an experiment to determine the refractive index of a glass block using a ray box, a student obtains two values: 1.52 and 1.48. Which of the following is a likely reason for the difference?
Medium
- AThe block was rotated between measurements
- BThe ray box battery was low
- CParallax error in marking ray positions
- DThe refractive index of air changed
20.Which type of lens is used to correct short-sightedness?
Easy
- AConverging lens
- BDiverging lens
- CConvex lens
- DPlano-convex lens
21.When light passes from air into glass, which of the following does not change?
Easy
- ASpeed
- BWavelength
- CFrequency
- DDirection
22.Which lens is used to correct long-sightedness?
Easy
- AConcave lens
- BDiverging lens
- CConvex lens
- DPlano-concave lens
23.In an experiment to find the critical angle of a glass block, a student increases the angle of incidence until the refracted ray disappears. The critical angle is measured as 42°. What is the refractive index of the glass? (sin 42° = 0.669)
Medium
- A1.49
- B0.669
- C1.00
- D2.40
24.A student investigates the position of the image in a plane mirror. She places an object pin 6.0 cm in front of the mirror. She then places a second pin behind the mirror and adjusts it until the images coincide. What distance behind the mirror should the second pin be placed?
Easy
- A3.0 cm
- B6.0 cm
- C12.0 cm
- D0.0 cm
25.In an experiment to determine the focal length of a converging lens, a student measures object distance u = 5.0 cm and image distance v = 7.5 cm. Using the equation 1/f = 1/u + 1/v, what is the focal length f?
Medium
- A3.0 cm
- B12.5 cm
- C2.0 cm
- D0.33 cm
26.What is the angle of incidence equal to according to the law of reflection?
Easy
- Aangle of refraction
- Bangle of reflection
- Ccritical angle
- D90°
27.In a plane mirror, the image is the same distance behind the mirror as the object is in front.
Easy
True or false?
28.A virtual image can be projected onto a screen.
Easy
True or false?
29.Total internal reflection can only occur when light travels from a medium of higher refractive index to a medium of lower refractive index. Which additional condition must be satisfied?
Medium
- AThe angle of incidence is less than the critical angle.
- BThe angle of incidence is greater than the critical angle.
- CThe angle of incidence equals the angle of reflection.
- DThe light must be monochromatic.
30.When an object is placed at a distance greater than 2F from a converging lens, the image formed is:
Medium
- Areal, inverted, and diminished
- Breal, inverted, and magnified
- Cvirtual, upright, and magnified
- Dreal, upright, and same size
31.Match each lens type to its correct use in correcting vision.
Medium
- Converging lens
- Diverging lens
- Corrects long-sightedness
- Corrects short-sightedness
32.A ray of light strikes a plane mirror, making an angle of 40° with the normal. What is the angle of reflection?
Easy
- A40°
- B50°
- C90°
- D140°
33.Which diagram correctly shows the dispersion of white light by a glass prism?
Medium
- AA: red at top, violet at bottom, both bending downwards
- BB: violet at top, red at bottom, both bending upwards
- CC: red at top, violet at bottom, both bending upwards
- DD: violet at top, red at bottom, both bending downwards
34.Which of the following are conditions for total internal reflection to occur? (Select all that apply)
Medium
- ALight travels from a more dense to a less dense medium
- BLight travels from a less dense to a more dense medium
- CThe angle of incidence is greater than the critical angle
- DThe angle of incidence is less than the critical angle
- EAll the incident light is reflected back into the first medium
35.Match each term with its correct description.
Medium
- Principal axis
- Principal focus
- Focal length
- The point where parallel rays converge or appear to diverge
- The distance from the lens centre to the principal focus
- A line through the centre of a lens perpendicular to its surface
36.Complete the sentence about refraction.
Easy
When light passes from air into glass, it bends ____ the normal.
37.What is the name of the line drawn at right angles to the boundary between two media in optics?
Easy
38.The refractive index of a material is 1.5. Calculate the critical angle for this material. (Give your answer in degrees to 1 decimal place.)
Medium
39.Using the diagram, calculate the refractive index of the glass. (sin 30° = 0.5, sin 19° = 0.3256)
Medium
40.A ray of light enters a glass block of refractive index 1.53 at an angle of incidence of 15°. Calculate the angle of refraction in degrees (to 1 decimal place).
Medium
41.Complete the sentence.
Easy
The angle of incidence is equal to the angle of ____.
42.State the two conditions required for total internal reflection to occur.
Medium
43.Complete the sentence about the visible spectrum.
Easy
In the visible spectrum, the colour with the longest wavelength is ____ and the colour with the shortest wavelength is ____.
44.State what is meant by the term 'critical angle'.
Easy
45.A ray of light passes from air into a glass block (refractive index 1.5) at an angle of incidence of 40°. Using Snell's law, calculate the angle of refraction. (sin 40° = 0.6428)
Medium
46.Explain why a diamond appears to sparkle more than a piece of glass of the same shape. The refractive index of diamond is 2.4 and of glass is 1.5.
Hard
47.The refractive index of a material is 1.5. Calculate the critical angle for this material. (Give your answer in degrees to 1 decimal place.)
Medium
- A37.6 °
- B50.2 °
- C41.8 °
- D46 °
48.Using the diagram, calculate the refractive index of the glass. (sin 30° = 0.5, sin 19° = 0.3256)
Medium
- A1.54
- B3.54
- C2.54
- D0.54
49.A ray of light enters a glass block of refractive index 1.53 at an angle of incidence of 15°. Calculate the angle of refraction in degrees (to 1 decimal place).
Medium
- A11.7 °
- B8.7 °
- C9.7 °
- D10.7 °
50.A ray of light passes from air into a glass block (refractive index 1.5) at an angle of incidence of 40°. Using Snell's law, calculate the angle of refraction. (sin 40° = 0.6428)
Medium
- A30.4 °
- B25.4 °
- C22.9 °
- D27.9 °

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