Light

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Reflection of Light

The law of reflection states: angle of incidence (i) = angle of reflection (r).
Angles are measured between the ray and the normal (an imaginary line perpendicular to the boundary).
In a plane mirror, the image is virtual, same size, same distance behind the mirror as the object is in front, and laterally inverted.
A virtual image is formed where extended reflected rays appear to meet; it cannot be projected onto a screen.
When drawing ray diagrams, use arrows to show direction: incident ray points toward the boundary, reflected ray points away.

Reflection at a plane mirror

Investigating Reflection

Aim: To investigate reflection by a plane mirror.
Independent variable: angle of incidence; dependent variable: angle of reflection.
Method: Use a ray box, protractor, and plane mirror; mark incident and reflected ray positions on paper.
Analysis: The law of reflection i = r should be verified; results are recorded in a table.
Safety: Avoid looking directly at the ray box light; run burns under cold water for at least five minutes.

Apparatus to investigate reflection

Refraction of Light

Refraction is the change in direction of a light ray when passing from one medium to another.
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.
When light enters a rectangular glass block, it bends towards the normal at entry and away at exit.
Only speed and wavelength change during refraction; frequency remains constant.

Refraction of light

Refractive Index

Refractive index n = speed of light in vacuum / speed of light in material (n > 1).
Also given by Snell's law: n = sin i / sin r, where i = angle of incidence, r = angle of refraction.
Denser materials have higher refractive indices (e.g., diamond ≈ 2.4, glass ≈ 1.5).
Use the inverse sine function (sin⁻¹) to find an angle from its sine value.

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.
The critical angle is the angle of incidence for which the angle of refraction is 90°.
When i > critical angle, all light is reflected back into the denser medium.
Optical fibres use TIR to transmit light signals for communications, endoscopes, and decorative lamps.

Refraction and total internal reflection compared

$Refraction and total internal reflection compared$

Ray Diagrams and Lenses

A converging (convex) lens brings parallel rays to a focus at the principal focus (focal point).
Focal length is the distance from the lens centre to the principal focus.
Three key rays: through optical centre (undeviated), parallel to principal axis (refracts through focus), through focus (emerges parallel).
For an object beyond 2f, the image is real, inverted, and diminished.
For an object between f and 2f, the image is real, inverted, and magnified.
For an object at 2f, the image is real, inverted, and same size.

Converging lens: principal focus and focal length

Virtual Images

A real image is formed where light rays converge; it can be projected onto a screen and is inverted.
A virtual image is formed where light rays appear to diverge from; it cannot be projected and is upright.
When an object is placed closer than the focal length to a converging lens, a virtual, upright, magnified image is formed (magnifying glass).
In ray diagrams for virtual images, use dashed lines to trace back the diverging rays to where they appear to meet.

Virtual image formed by a magnifying glass

Dispersion of Light

Dispersion is the separation of white light into its component colours by a prism.
Violet light is refracted most; red light is refracted least.
The visible spectrum in order of increasing wavelength: violet, indigo, blue, green, yellow, orange, red.
Each colour corresponds to a different wavelength and frequency; frequency does not change during refraction.

Dispersion of white light through a prism

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Practice questions

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1.What is the angle between the incident ray and the normal called?
Easy
- Aangle of reflection
- Bangle of incidence
- Ccritical angle
- Drefractive angle
2.The angle of incidence is always equal to the angle of reflection.
Easy
True or false?
3.State the law of reflection.
Easy
4.Complete the sentence:
Easy
When light passes from air into glass, it bends ____ the normal.
5.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. (Give your answer to 1 decimal place.)
Medium
- A10.7
- B8.7
- C9.7
- D11.7
6.Which colour of light is refracted the most when white light passes through a prism?
Easy
- Ared
- Bgreen
- Cviolet
- Dyellow
7.What is the name of the angle at which the angle of refraction is 90°?
Easy
8.Total internal reflection can occur when light travels from a less dense medium to a denser medium.
Medium
True or false?

More questions

9.Match each term with its correct description.
Medium
- real image
- virtual image
- principal focus
- can be projected onto a screen
- cannot be projected onto a screen
- point where parallel rays converge
10.Name one practical application of total internal reflection.
Easy
11.In a ray diagram for reflection, what does the normal line represent?
Easy
- Athe mirror surface
- Ba line at 90° to the mirror
- Cthe incident ray
- Dthe reflected ray
12.A ray of light strikes a plane mirror at an angle of incidence of 40°. What is the angle of reflection? (Give your answer in degrees.)
Medium
13.The law of reflection states that the angle of incidence is equal to the angle of reflection.
Easy
True or false?
14.What is the name of the imaginary line drawn at right angles to the boundary between two media?
Easy
- Anormal
- Bprincipal axis
- Cfocal length
- Drefracted ray
15.Which of the following is a property of the image formed in a plane mirror?
Medium
- Areal and inverted
- Bvirtual and laterally inverted
- Creal and upright
- Dvirtual and diminished
16.When light travels from a less dense medium to a denser medium, it bends away from the normal.
Easy
True or false?
17.What is the critical angle defined as?
Medium
- AThe angle of incidence at which the angle of refraction is 90°
- BThe angle of refraction when light enters a denser medium
- CThe angle at which light is totally reflected back into the same medium
- DThe angle between the incident ray and the reflected ray
18.Which of the following is a use of total internal reflection?
Easy
- Amagnifying glass
- Bplane mirror
- Coptical fibres
- Dprism spectacles
19.A real image can be projected onto a screen.
Easy
True or false?
20.Which colour of light has the longest wavelength in the visible spectrum?
Easy
- Aviolet
- Bblue
- Cred
- Dgreen
21.A ray of light passes from air into a glass block. The refractive index of glass is 1.5. If the angle of incidence is 30°, what is the angle of refraction? (sin30° = 0.5, sin19.5° = 0.333)
Medium
- A19.5°
- B30°
- C45°
- D10°
22.Arrange the colours of the visible spectrum in order of increasing wavelength (shortest to longest).
Medium
- Violet
- Blue
- Green
- Yellow
- Orange
- Red
23.Which of the following are properties of the image formed by a plane mirror? (Select all that apply)
Medium
- ASame size as the object
- BLaterally inverted
- CReal
- DSame distance behind the mirror as the object is in front
24.When light travels from air into glass, it bends away from the normal.
Easy
True or false?
25.A ray of light passes from a liquid into air. The refractive index of the liquid is 1.47. The angle of incidence in the liquid is 35°. What is the angle of refraction in air? (sin 35° = 0.574)
Hard
- A22.9°
- B35.0°
- C57.5°
- D90.0°
26.An object is placed beyond 2F from a converging lens. What type of image is formed?
Medium
- AReal, inverted and diminished
- BReal, upright and magnified
- CVirtual, upright and magnified
- DReal, inverted and the same size as the object
27.Using the diagram, calculate the refractive index of the glass. (sin 30° = 0.500, sin 20° = 0.342)
Medium
- A1.46
- B1.71
- C0.68
- D2.46
28.White light is passed through a glass prism. Which colour is refracted the most?
Easy
- ARed
- BGreen
- CBlue
- DViolet
29.A student investigates reflection using a plane mirror. The angle of incidence is measured as 30°. What should the angle of reflection be according to the law of reflection?
Easy
- A30°
- B60°
- C15°
- D90°
30.A ray of light passes from glass into air. The critical angle for the glass-air boundary is 42°. What happens when the angle of incidence in the glass is 50°?
Medium
- ATotal internal reflection occurs
- BThe ray is refracted away from the normal
- CThe ray is refracted along the boundary
- DThe ray passes straight through without changing direction
31.A converging lens has a focal length of 10 cm. An object is placed 25 cm from the lens. Which of the following best describes the image formed?
Medium
- AReal, inverted, diminished
- BReal, upright, magnified
- CVirtual, inverted, same size
- DVirtual, upright, diminished
32.In an experiment to verify the law of reflection using a ray box and a plane mirror, which of the following is the most important control variable?
Medium
- ADistance of the ray box from the mirror
- BAngle of incidence
- CAngle of reflection
- DThickness of the mirror
33.Which of the following is a practical difficulty when using the pin method to determine the refractive index of a transparent block?
Medium
- AThe pins may not be vertical
- BThe block may be too transparent
- CThe angle of incidence is always 30°
- DThe refractive index changes with temperature
34.A student obtains two values for the refractive index of a glass block: 1.49 and 1.55. Which of the following is the most likely reason for the difference?
Medium
- AThe block was not placed on the same part of the paper each time
- BThe ray box battery was low
- CThe angle of incidence was measured from the mirror instead of the normal
- DThe student used different coloured light
35.In the pin method for determining refractive index, the pins should be placed as close together as possible to improve accuracy.
Medium
True or false?
36.Match each experimental term with its correct description.
Medium
- Independent variable
- Dependent variable
- Control variable
- Angle of reflection
- Angle of incidence
- Distance of ray box from mirror
37.Arrange the following steps in the correct order for determining the refractive index of a glass block using the pin method.
Medium
- Place the glass block on paper and draw its outline.
- Draw the incident ray at a known angle of incidence.
- View through the block and place pins to mark the emergent ray.
- Remove the block and draw the emergent ray line.
- Measure the distances a and b to calculate n.
38.Which of the following are possible sources of error when using the pin method to measure refractive index? (Select all that apply)
Medium
- AParallax error when viewing pins
- BThe block is not perfectly rectangular
- CThe ray box produces white light
- DThe pins are not exactly vertical
39.In the dispersion of white light by a prism, which colour is refracted the least?
Easy
- ARed
- BOrange
- CGreen
- DViolet
40.In a plane mirror, the image is the same size as the object.
Easy
True or false?
41.What is the name of the line drawn at right angles to the boundary between two media in optics?
Easy
- ANormal
- BIncident ray
- CReflected ray
- DPrincipal axis
42.Which of the following is a property of the image formed by a plane mirror?
Easy
- AReal and inverted
- BVirtual and laterally inverted
- CReal and upright
- DVirtual and magnified
43.Total internal reflection can only occur when light travels from a denser medium to a less dense medium.
Medium
True or false?
44.An object is placed at a distance greater than 2F from a converging lens. Which of the following describes the image formed?
Medium
- AReal, inverted, diminished
- BReal, upright, magnified
- CVirtual, inverted, same size
- DVirtual, upright, magnified
45.Arrange the following colours in order of increasing wavelength (shortest to longest).
Easy
- Blue
- Red
- Green
- Violet
46.Complete the sentence about dispersion.
Easy
When white light passes through a glass prism, the colour that is refracted the most is ____.
47.Fill in the blank: In a converging lens, rays of light that are parallel to the principal axis are refracted to pass through the ___.
Easy
In a converging lens, rays of light that are parallel to the principal axis are refracted to pass through the ____.
48.Light travels from air into a diamond. The refractive index of diamond is 2.42. If the angle of incidence is 30°, calculate the angle of refraction. (Give your answer in degrees to 1 decimal place.)
Hard
49.Complete the sentence about total internal reflection.
Easy
Total internal reflection can only occur when light travels from a ____ medium to a ____ medium.
50.State one practical application of total internal reflection.
Easy
51.Light travels from air into a glass block. The refractive index of the glass is 1.5. The angle of incidence is 30°. Calculate the angle of refraction. Use the formula n = sin i / sin r. (Give your answer to 1 decimal place.)
Medium
52.In a refraction experiment, a student measures a = 3.2 cm and b = 6.9 cm for i = 45°. The refractive index is given by n = 0.71 × (b/a). Calculate n.
Medium
53.Describe how you would use a ray box and a plane mirror to verify the law of reflection. Include the measurements you would take.
Medium
54.Explain what is meant by the critical angle when light travels from a denser to a less dense medium. Include a diagram in your explanation.
Hard
55.Light travels from air into a diamond. The refractive index of diamond is 2.42. If the angle of incidence is 30°, calculate the angle of refraction. (Give your answer in degrees to 1 decimal place.)
Medium
- A11.9 °
- B10.9 °
- C12.9 °
- D13.9 °
56.Light travels from air into a glass block. The refractive index of the glass is 1.5. The angle of incidence is 30°. Calculate the angle of refraction. Use the formula n = sin i / sin r. (Give your answer to 1 decimal place.)
Medium
- A18.5 °
- B21.5 °
- C19.5 °
- D20.5 °
57.In a refraction experiment, a student measures a = 3.2 cm and b = 6.9 cm for i = 45°. The refractive index is given by n = 0.71 × (b/a). Calculate n.
Medium
- A3.53
- B1.53
- C0.53
- D2.53

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