Optics Flashcards

All 39 cards in this deck

How does a lens form an image?

By refracting light.

What is the principal focus of a convex lens?

The point where rays of light parallel to the axis are brought to a focus after passing through the lens.

What is the focal length of a lens?

The distance from the lens to the principal focus.

What is the ray diagram symbol for a convex lens and for a concave lens?

Convex: a vertical line with arrowheads pointing outwards at each end.
Concave: a vertical line with arrowheads pointing inwards at each end.

What does a concave lens do to rays of light that are parallel to the axis?

It makes them diverge, so they appear to come from the principal focus on the same side as the object.

True or false? The image produced by a concave lens is always virtual.

True. A concave lens diverges light, so the image is always virtual (and upright and diminished).

True or false? A convex lens can only produce a real image.

False. A convex lens produces a real image or a virtual image, depending on where the object is placed.

Give three words that describe the image formed by a concave lens.

Virtual, upright and diminished.

In a ray diagram, what shows that an image is virtual?

The refracted rays have to be traced backwards (dashed construction lines) to where they appear to meet.

How does the size of the image change as an object moves further away from a concave lens?

It decreases.

True or false? A graph of image distance against object distance for a convex lens shows the two quantities are inversely proportional.

False. The image distance decreases as object distance increases, but the relationship is not inverse proportionality.

What is the equation for the magnification produced by a lens?

$\text{magnification} = \dfrac{\text{image height}}{\text{object height}}$

What are the units of magnification?

None — magnification is a ratio, so it has no units.

In a magnification calculation, what units should image height and object height be measured in?

Both in the same unit — either both in mm or both in cm.

What are the steps to find the magnification of a lens from a ray diagram?
e.g. a diagram where the object is $2$ cm tall and the image is $6$ cm tall

1. Measure the object height with a ruler: $2$ cm.
2. Measure the image height in the same unit: $6$ cm.
3. Divide image height by object height: magnification $= 6 \div 2 = 3$.

What are the steps to find the image height from the magnification?
e.g. magnification $3$, object height $2$ cm

1. Rearrange: image height $=$ magnification $\times$ object height.
2. Substitute: $3 \times 2$.
3. Image height $= 6$ cm.

True or false? A magnification of less than 1 means the image is diminished.

True. The image height is smaller than the object height.

True or false? Magnification is measured in cm.

False. Magnification is a ratio of two heights, so it has no units.

What does each colour in the visible light spectrum have?

Its own narrow band of wavelength and frequency.

Which colour of visible light has the longest wavelength, and which has the shortest?

Red has the longest wavelength; violet has the shortest.

List the colours of the visible spectrum in order from longest to shortest wavelength.

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

Which colour of visible light has the highest frequency?

Violet — it has the shortest wavelength.

True or false? Red light has a higher frequency than blue light.

False. Red has the longest wavelength and so the lowest frequency of the visible colours.

True or false? White light is a mixture of all the colours of the visible spectrum.

True. It contains all the bands of wavelength and frequency of visible light.

What name is given to reflection from a smooth surface in a single direction?

Specular reflection.

What name is given to reflection from a rough surface, which causes scattering?

Diffuse reflection.

Which type of reflection produces a clear image, and why?

Specular reflection — the smooth surface reflects the light in a single direction.

True or false? In diffuse reflection the law of reflection is not obeyed.

False. Each ray still reflects with angle of reflection equal to angle of incidence, but the rough surface points the normals in different directions, so light is scattered.

True or false? A sheet of white paper reflects light diffusely because its surface is rough.

True. The rough surface scatters the light in all directions.

What determines the colour of an opaque object?

Which wavelengths of light it reflects most strongly; the wavelengths that are not reflected are absorbed.

Why does an opaque object appear white, and why does one appear black?

White: all wavelengths are reflected equally. Black: all wavelengths are absorbed.

How does a colour filter work?

It absorbs certain wavelengths (colours) and transmits others.
e.g. a red filter transmits only red light.

What colour does a blue opaque object appear when lit only by red light?

Black — the red light is absorbed by the object, so no light is reflected.

What are the two words used for objects that transmit light, and what is the word for one that does not?

Transparent and translucent transmit light; opaque does not transmit light.

What is the difference between a transparent and a translucent object?

A transparent object transmits light so things can be seen clearly through it; a translucent object transmits light but scatters it, so things cannot be seen clearly.

True or false? A white object viewed through a green filter appears green.

True. The white object reflects all wavelengths, but the filter transmits only green light to the eye.

True or false? A red object appears red because it absorbs red light.

False. It reflects red light most strongly and absorbs the other wavelengths.

What are the steps to locate the image in a convex lens ray diagram?

e.g. an object standing beyond the focal length

1. Ray from the top of the object parallel to the axis → refracts through the principal focus.
2. Ray from the top of the object through the centre of the lens → straight on.
3. Image is where the two rays cross: real and inverted.

Where must an object be placed for a convex lens to form a real image?

Further from the lens than the focal length (beyond the principal focus).