Defects of vision and their correction using lenses (A-level only) Flashcards – AQA A-level Physics (7408)

What happens to rays parallel to the principal axis when they pass through a converging lens?

They converge at a real principal focus.

What happens to rays parallel to the principal axis when they pass through a diverging lens?

They spread out as if originating from a virtual principal focus.

What point on a lens does its principal axis pass through?

The optical centre.

What is the focal length of a lens?

The distance from the lens to its principal focus.

Under the usual sign convention, what are the focal-length signs for converging and diverging lenses?

Converging: positive; diverging: negative.

How is lens power calculated from focal length, and what units are used?

$P = \frac{1}{f}$ where $f$ is in metres and $P$ is in dioptres (D).

What are the signs of the powers of converging and diverging lenses?

Converging: positive; diverging: negative.

How does the magnitude of lens power relate to the magnitude of focal length?

A larger power magnitude corresponds to a shorter focal-length magnitude.

In a ray diagram, how is a ray through the optical centre of a lens approximated?

As undeviated.

Why are distant objects unclear to a person with myopia?

In a relaxed eye, light from a distant object focuses in front of the retina.

What are two possible causes of myopia related to the eye’s shape or optical power?

The eye may be too long or its optical power too great.

What is the far point of an eye?

The greatest distance at which an object can be seen clearly without correction.

How does a diverging lens correct myopia?

It makes incoming light diverge so the eye can focus it on the retina.

Where does a correcting lens form the image of a distant object for a myopic eye?

It forms a virtual image at the eye’s far point.

For a thin correcting lens close to the eye, a very distant object and a far point at distance $R$ metres, what are the image distance, focal length and required lens power?

$v=-R,\quad f=-R,\quad P=-\frac{1}{R}\text{ dioptres}$

What is hypermetropia (long-sightedness)?

Difficulty focusing on nearby objects.

Where would light from a near object focus in a hypermetropic eye with insufficient convergence?

Behind the retina.

What are two possible causes of hypermetropia?

The eye may be too short or have too little optical power.

What is a person's near point?

The closest distance at which they can see an object clearly without correction.

How does a converging lens help a hypermetropic eye focus a near object on the retina?

It increases the convergence of light entering the eye.

Where can a correcting lens form a virtual image of an object at the desired viewing distance for a hypermetropic person?

At the person's near point, where the eye can focus it.

For a correcting lens, what is the image distance $v$ for a virtual image at near-point distance $N$?

$v=-N$

What lens power is required to correct hypermetropia for an object at distance $u$ and a near point at distance $N$? Assume a thin lens close to the eye and distances in metres.

$P=\frac{1}{f}=\frac{1}{u}-\frac{1}{N}$ in dioptres.

What is astigmatism?

A condition in which the eye has different optical powers in different meridians.

What commonly causes the different optical powers in astigmatism?

The cornea or lens has different curvatures in different directions.

How does astigmatism affect focus and vision?

Light is not brought to a single sharp focus in the same way in all directions, producing blurred or distorted vision.

How does cylindrical correction compensate for astigmatism?

It supplies different optical power in different meridians.

What does SPH specify in a lens prescription?

The spherical lens power, in dioptres.

What does CYL specify in a lens prescription?

The additional cylindrical power, including its sign, in dioptres.

What does the axis specify in a lens prescription?

The meridian along which the cylindrical component adds no power, stated in degrees from $0^\circ$ to $180^\circ$.

What are the powers in the two principal meridians of a lens prescription?

$\mathrm{SPH}$ and $\mathrm{SPH} + \mathrm{CYL}$.