Refraction

More Wave Properties of Light

Define refraction.

The change in direction of a wave caused by a change in wave speed when it passes from one medium into another.

What happens to the speed of a wave when it passes from one medium to another?

The wave speed changes.

What happens to the frequency of a wave during refraction?

The frequency remains constant.

What happens to the wavelength of a wave during refraction?

The wavelength changes because the wave speed changes while frequency remains constant.

Why does a wave change direction during refraction?

Different parts of the wavefront enter the new medium at different times and change speed, causing the wavefront to bend.

When will a wave not change direction as it enters a different medium?

When it travels along the normal, so it enters the boundary at 90°.

Why does a wave travelling along the normal not change direction?

All parts of the wavefront enter the new medium at the same time and change speed together.

What is the normal?

An imaginary line perpendicular to the boundary between two media at the point where the ray meets the boundary.

What is the angle of incidence?

The angle between the incident ray and the normal.

What is the angle of refraction?

The angle between the refracted ray and the normal.

Are angles in refraction measured from the surface or the normal?

From the normal.

What happens to light when it enters a medium in which it travels more slowly?

It bends towards the normal.

What happens to light when it enters a medium in which it travels faster?

It bends away from the normal.

What happens to light travelling from air into glass?

It slows down and bends towards the normal.

What happens to light travelling from glass into air?

It speeds up and bends away from the normal.

What happens to wavelength when a wave slows down?

The wavelength decreases.

What happens to wavelength when a wave speeds up?

The wavelength increases.

Why does wavelength decrease when a wave slows down?

From v = fλ, frequency stays constant, so a lower speed requires a shorter wavelength.

What wave equation can be used when considering refraction?

v = fλ.

Define refractive index.

A measure of how much a material changes the speed of light compared with its speed in a vacuum.

What is the symbol for refractive index?

n.

What equation defines refractive index?

n = c/v.

In n = c/v, what does c represent?

The speed of light in a vacuum.

In n = c/v, what does v represent?

The speed of light in the material.

Does refractive index have a unit?

No, refractive index is dimensionless.

What is the approximate refractive index of air?

1.00.

What does a larger refractive index mean about the speed of light in a material?

Light travels more slowly in the material.

What does a smaller refractive index mean about the speed of light in a material?

Light travels faster in the material.

How can the speed of light in a material be calculated from its refractive index?

v = c/n.

State Snell's law.

n₁ sin θ₁ = n₂ sin θ₂.

In Snell's law, what are n₁ and n₂?

The refractive indices of medium 1 and medium 2.

In Snell's law, what are θ₁ and θ₂?

The angles the ray makes with the normal in medium 1 and medium 2.

What must you remember when using Snell's law?

All angles must be measured from the normal.

How can Snell's law be rearranged to find θ₂?

sin θ₂ = (n₁ sin θ₁)/n₂, then θ₂ = sin⁻¹[(n₁ sin θ₁)/n₂].

If n₂ is greater than n₁, how does the ray bend?

Towards the normal.

If n₂ is less than n₁, how does the ray bend?

Away from the normal.

Why does light bend towards the normal when entering a higher refractive index material?

Its speed decreases.

Why does light bend away from the normal when entering a lower refractive index material?

Its speed increases.

What happens if a light ray enters a glass block exactly along the normal?

Its speed and wavelength change, but its direction does not.

How can the refractive index of a material be measured experimentally?

Measure several angles of incidence and their corresponding angles of refraction and use Snell's law.

Why should several different angles be measured when determining refractive index?

They allow a graph to be plotted and reduce the effect of random error.

Why can a semicircular glass block be useful for measuring refractive index?

A ray directed through the centre meets the curved surface along a radius, so it travels along the normal and does not refract at that surface.

For light travelling from air into a material, what does Snell's law become if n_air ≈ 1?

sin θ₁ = n₂ sin θ₂.

For light travelling from air into a material, how can refractive index be calculated?

n = sin θ₁ / sin θ₂.

What graph can be plotted to determine refractive index experimentally?

Plot sin θ₂ on the y-axis against sin θ₁ on the x-axis.

What should the graph of sin θ₂ against sin θ₁ look like?

A straight line through the origin.

If sin θ₂ is plotted against sin θ₁ for light entering from air, what is the gradient?

1/n.

How can refractive index be found from the gradient of a sin θ₂ against sin θ₁ graph?

n = 1/gradient.

What is dispersion?

The separation of light into different wavelengths or colours because different wavelengths are refracted by different amounts.

How can a glass prism produce a spectrum?

Different wavelengths of white light are refracted by different amounts, separating the light into its component colours.

Why do different colours refract by different amounts?

The refractive index of glass depends on wavelength, so different wavelengths travel at different speeds in the glass.

Which visible colour has the shortest wavelength?

Violet.

Which visible colour has the longest wavelength?

Red.

Which visible colour is refracted most strongly by glass?

Violet.

Which visible colour is refracted least strongly by glass?

Red.

Which colour has the greatest refractive index in glass?

Violet.

Which colour has the smallest refractive index in glass?

Red.

Why does violet light bend closer to the normal than red light when entering glass?

Violet has a shorter wavelength and a larger refractive index in glass, so it is refracted more strongly.

What happens to different colours when they leave a prism?

They refract by different amounts again, increasing their separation into a spectrum.

What order are visible colours in a typical spectrum from longest to shortest wavelength?

Red, orange, yellow, green, blue, indigo, violet.

Does the frequency of each colour change when light enters glass?

No, frequency remains constant.

Does the wavelength of light change when it enters glass?

Yes, it decreases because the light slows down.

Why can refraction cause optical illusions?

Light changes direction at boundaries, so objects can appear to be in a different position from their actual position.

What is the principle of reversibility of light?

A light ray follows exactly the same path if its direction of travel is reversed.

What safety precaution is needed when using a ray box?

The ray box may become hot, so avoid touching hot parts.

What safety precaution is needed when handling glass blocks?

Do not knock them together because sharp chips may break off.