Lenses Chapter 5 Flashcards
Is a convex lens converging or diverging
Converging
Is a concave lens converging or diverging
Diverging
Describe what a convex lens looks like
Thick at the center and thinner at the sides.
Describe what a concave lens looks like
Thicker at the ends than in the centre (caves in)
What is the optic centre
Centre of the lens
What is the principal axis
Straight line that passes through the optic centre and hits the lens at ninety degrees
What is the focus
Point to which parallel beams to the axis converge to or appear to have converged from (for concave)
There are two focuses one for each side of lens
What is the focal length
Distance from the optic centre to the focus
How does a lens refract a ray which strikes the optic centre
It passes straight through the mirror
How does a lens refract a ray which strikes the lens travelling parallel to the axis
It travels through the focus on the other side of the lens
How does a lens refract a ray which strikes the lens after having passes through the focus
it emerges parallel to the principal axis
Explain real image
Image formed by the actual intersection of light rays
Explain virtual image
Image formed by the apparent intersection of light rays
Explain what an image will look like for a convex lens outside the focus (between 2F and F)
Image is real, on the other side of the lens to the object and it is inverted and magnified
Explain what an image will look like for a convex lens outside the focus (at 2F)
Image is real, on the other side of the lens to the object, inverted and same size
Explain what an image will look like for a convex lens outside the focus (Outside 2F)
Image is real, on the other side of the lens to the object, inverted and diminished
Explain what an image will look like for a convex lens at the focus
Image is virtual but at your blind spot. You will never see this image.
Explain what an image will look like for a convex lens inside the focus
Image is virtual and on the same side of the lens a the object. Image is erect and magnified.
Give two uses of convex lenses
microscopes and magnifying glasses
What is the formula for magnification
v/u - image height/object height
What is the formula to use for a convex mirror
1/u + 1/v = 1/f
How can you tell from the formula if an image is real or virtual and formed in a convex or concave lens
v is + for real image
v is - for virtual image
f is + for convex
f is - for concave
What is u in formula
distance from optic centre to object
What is v in formula
distance from optic centre to image
Explain how light refracts by a concave lens when it strikes the optic centre
Ray passes straight through the lens
Explain how light refracts by a concave lens when ray is heading for the focus when it strikes the lens
It emerges parallel to the principal axis
Explain how light refracts by a concave lens that is travelling parallel to the axis
It refracts as if it came from the focus.
Describe an image in a concave mirror
Virtual, on the same side of the lens as the object, erect and diminished
How does the distance from the concave effect the image
The nearer the object to the lens the bigger it is.
What is the formula for a concave lens
1/u - 1/v = -1/f
What is the relationship with the focal length of a lens and it’s power
a short focal length has a greater converging effect on a light beam and therefore has a larger power than a longer focal length
What is the formula for the power of a lens
1/focal length
Is the power of the concave lens positive or negative
negative
Is the power of the convex lens positive or negative
positive
What is the unit of the power of the lens
per metre (m to the power of -1)
How is the power of a combination of lenses found
P=P1+P2
sums of the powers
How is the focal length of a combination found
1/f = 1/f1 + 1/f2
where f is positive for a convex lens
where f is negative for a concave lens
What is the iris
The iris controls the amount of light entering the eye by making the pupil larger or smaller
Explain the focusing system of the eye
The cornea and crystalline lens form a converging system. The aqueous and vitreous humour (liquids) also form part of the system to focus light onto the back of the eye
What is the retina
a light sensitive screen at the back of the eye
How does the retina allow us to see
Light strikes the retina and electrical messages are sent to the brain by the optic nerve. Results in sight. If image focuses in front or behind the retina the image is blurred
What is the power of accommodation
Ability to focus a real image on the retina
How does the eye focus an image
The ciliary muscles change the shape of the lens and hence the focal length. Object viewed from a large distance in a normal eye the object is viewed with the ciliary muscles relaxed. Lens is thinnest with greatest focal length
Explain least distance of distinct vision
Near point- the smallest distance between an object and the eye where the object is seen clearly without strain.
Explain short sighted
A short sighted person can see nearby objects clearly but cannot bring distant objects into focus
Explain long sighted
A long sighted person can see distant objects clearly but cannot bring nearby objects into focus
What lens corrects short sightedness
concave
What lens corrects long sightedness
convex
Use of a concave lens
Binoculars, telescopes, high quality cameras
use of convex lens
binoculars, refracting telescopes, magnification
