Optics Flashcards
What does a convex/converging lens do
It focuses parallel light. The focal length of a convex lens is the distance from the middle of the lens to the point where the light is focused
What is a concave/diverging lens
It spreads out parallel light. The light appears to come from a point. The focal length is from the middle of the lens to where the light appears to come from
How to find the focal length of a concave lens:
- get the lens and a flat white surface
- Find a distant light source and focus an image on the screen
- Move the screen and lens until the image is in focus
- Measure the distance from the lens to the screen to give the focal length
If the focal length is negative what lens is it
Concave / diverging
In a camera what lens is used to produce an image on a film
Converging
When an object is 5 m away, the image is:
- smaller
- inverted
- real
When the object is 25cm, the image is
- larger
- upright
- virtual
When the object is 75cm away the image is
- real
- inverted
- smaller
What does real mean
The image can be projected onto a screen. The image distance will be positive and the opposite side to the object
What does virtual mean
The image cannot be projected onto a screen. The image appears to be where the Ray is coming from. The image distance will be negative and the same side as the object
What do refracting telescopes do
Combine lenses to form an image of a distant object
How does a telescope work
- The telescope is aimed at an object and light is reflected off the object
- The light is refracted by the objective lens. The lens is convex so it focuses the light. The lens is large so a lot of light can enter it
- The light is refracted and magnified by the eye piece lens which is used by an observer to view the image
What are issues with making a telescope
- making a good lens smooth and clear
- making them large
- weight
A simple refracting telescope uses
- an objective lens that is convex
* a lens as an eyepiece that is convex
What does the image look like through a refracting telescope
- vertically and horizontally inverted (upside down)
- real
- larger
How does the image look through a reflecting telescope
- real
- larger
- vertically and horizontally inverted
What do reflecting telescopes do
Combine lenses and mirrors to form an image of a distant object
What does a simple reflecting telescope use
- primary concave mirror that reflects and focuses the light
- secondary plane mirror that changes the direction of the light by 90 degrees
- eyepiece is always a convex lens to magnify the image
Issues with making a reflecting telescope
- making a smooth mirror
- making a good lens, smooth and clear
- making them large
Benefits and disadvantages of reflecting and refracting telescopes
- refracting combine lenses to form an image of a distant object
- reflecting combine mirrors and lenses to form an image of a distant object
- refracting telescopes have a convex lens that is large to focus the light, making a lot of light enter
- disadvantages of refractive include it is hard to make the lens large, and they are heavy
- reflective telescopes have brighter images.
- it is easier to make larger mirrors
- refracting are easy to build, but have a dimmer light as light reflects of the lens
How does the eye work
- Light passes through the cornea
- The pupil controls the amount of light let in
- The lens refracts the light. The ciliary muscle controls the shape of the lens
- The refracted light forms an image on the back part of the eye called the retina
- The light is converted into electrical impulses and is sent along the optic nerve to the brain
- You ‘see’ the object
What does the ciliary muscle do
Change the size of the lens. This will bring into focus objects that are different distances away
What is short sightedness
It occurs when the image forms before it reaches the retina. Lenses can be used to refract the light in the opposite direction before it reaches the eye.
What lenses are used to correct short sightedness
Concave lenses. They have a negative power
What are used to correct long sightedness
Convex lenses. They have a positive power
What is long sightedness
It occurs when the image forms after it reaches the retina. It is when the eyeball is too short
What is the far point
It is the furthest an object can be from the eye without it appearing blurred. The human far point is infinity.
What is the human near point
Closest an object can be from the eye without being blurred. The human near point is 25cm
Where can lenses be found
- eyes
- microscopes
- telescopes
- glasses
- cameras
