3.2 Light (Cambridge (CIE) IGCSE Physics)

The dependent variable in the reflection investigation is the angle of reflection.

In an experiment to investigate reflection by a plane mirror the equipment needed is:

• a ray box

• a protractor

• a sheet of paper

• a pencil

• a ruler

True.

Looking directly into a ray box light may damage the eyes.

In an experiment to investigate reflection by a plane mirror, the results you would expect to find are that the angle of reflection is equal to the angle of incidence at all angles tested.

If the light ray box causes burns, the burn should be held under cold running water for at least five minutes.

In an experiment to investigate reflection by a plane mirror, control variables include:

• distance of the ray box from the mirror

• width of the light beam

• frequency/wavelength of the light

True.

Systematic errors are errors in the use of apparatus or the experimental method. Errors in measuring angles due to drawing normal lines incorrectly are systematic errors.

True.

Using a sharp pencil and marking the centre of the beam to then draw the ray will reduce random errors in the results.

The symbol for the angle of incidence of a light ray on a boundary is .

The symbol for the angle of refraction of a light ray on a boundary is .

False.

When light refracts by passing from a less dense medium to a more dense medium, it bends towards the normal.

False.

The frequency of light does not change during refraction.

True.

When light passes along the normal, perpendicular to the surface of a medium, the ray does not change direction.

False.

When light refracts by passing from a denser medium into a less dense medium, the ray bends away from the normal.

During refraction, the speed and wavelength of light change, but not the frequency.

In a ray diagram for refraction through a glass block, the ray passes through 2 boundary surfaces:

• air-to-glass

• glass-to-air

True.

When light refracts through a glass block, the first boundary is air-to-glass, and the second boundary is glass-to-air. The refracted ray at the air-to-glass boundary becomes the incident ray at the glass-to-air boundary.

The refractive index is the ratio of the speed of light in a vacuum and the speed of light in a material.

True.

Since the refractive index is a ratio, it has no units.

False.

When , the refracted ray bends away from the normal.

The equation for the refractive index is

Where:

• = the refractive index

True.

The value for the refractive index of a material will always be less than the speed of light in a vacuum.

The trigonometric function used in Snell's Law is sine.

True.

When , the refracted ray bends toward the normal.

The equation for Snell's law in terms of the refractive index is

Where:

• = refractive index of the material, which has no units

• = angle of incidence, measured in degrees (°)

• = angle of refraction, measured in degrees (°)

The critical angle is the angle of incidence that gives an angle of refraction of exactly 90°.

Total internal reflection (TIR) occurs when the angle of incidence is greater than the critical angle, and the ray is travelling from a denser material into a less dense material.

Total internal reflection is used to transmit information through optical fibre cables for television and internet communications.

The two conditions required for total internal reflection to occur are:

• angle of incidence > critical angle

• ray is travelling from a denser material to a less dense material

The equation for the critical angle in terms of the refractive index is

Where:

• = refractive index, which has no units

• = critical angle, measured in degrees (°)

True.

In a periscope, two right-angled prisms utilise total internal reflection to reflect light.

The refractive index is a measure of the change in the speed of light as it travels from a vacuum into a material. The refractive index is a ratio, so it has no units, and its value is always greater than 1.

False.

Total internal reflection only occurs when light travels from a denser material into a less dense material.

The principal axis is a line which passes through the centre of a lens.

The principal focus or focal point, is the point at which rays of light travelling parallel to the principal axis intersect the principal axis and converge at the point where the diverging rays appear to proceed.

Focal length is the distance between the centre of the lens and the principal focus.

The two types of lenses are:

• converging lenses (convex)

• diverging lenses (concave)

True.

In a converging lens, parallel rays are brought to a focus at a point called the principal focus.

False.

The more curved the converging lens is, the shorter the focal length.

True.

In a diverging lens, parallel rays are made to diverge (spread out) from a point called the principal focus.

The two types of images that can be produced by lenses are:

• real images

• virtual images

True.

Real images are always inverted.

A real image is an image that is formed when the light rays from an object converge and meet each other and can be projected onto a screen.

False.

Real images can be projected onto a screen.

A real image is produced by the convergence of light toward a focus.

True.

Virtual images are always upright.

A virtual image is an image that is formed when the light rays from an object do not meet but appear to meet behind the lens. Virtual images cannot be projected onto a screen.

False.

Virtual images cannot be projected onto a screen.

Virtual images are formed by the divergence of light away from a point.

False.

A converging lens will form a real image of an object if the object is placed at a distance greater than the focal length of the lens.

True.

A converging lens will form a virtual image of an object if the object is placed at a distance less than the focal length of the lens.

True.

Both converging and diverging lenses are commonly used to correct defects of sight.

True.

Short-sighted vision is caused by the eye refracting light and bringing it to a focal point in front of the retina.

Converging lenses are used to correct long-sighted vision.

Diverging lenses are used to correct short-sighted vision.

People with short-sighted vision can only see objects which are close to them, they can not see objects which are far away.

People with long-sighted vision can only see objects which are far away, they cannot see objects which are close to them.

True.

Long-sighted vision is caused by the eye refracting light and bringing it to a focal point behind the retina.

False.

Convex or converging lenses are used to correct the vision of long-sighted people.

True.

White light contains all the colours of the visible spectrum.

Visible light is a small part of the electromagnetic spectrum.

A prism can be used to split white light into its constituent colours when light is passed through it.

The process of splitting white light into its constituent colours is called dispersion.

Dispersion is caused by the refraction of light through a prism. Red light is refracted the most and violet light is refracted the least.

The seven colours that make up white light are:

• red

• orange

• yellow

• green

• blue

• indigo

• violet

Red light has the longest wavelength of all the colours in the visible spectrum.

False.

Blue light has a shorter wavelength than yellow light.

False.

Orange light has a lower frequency (and a longer wavelength) than violet light.

Monochromatic light is light with a single frequency (or wavelength).