Snell's law determines how sharply the rays bend
Any ray passing through the near focal point emerges as a parallel ray
The focal length of a converging lens is:
A. the distance from the object where an image is formed
B. the distance at which an object must be placed to form an image
C. the distance from the lens plane at which parallel light rays are focused
D. the distance from the front surface of the lens to its back surface
E. the point at which light is focused inside the lens
A real image (left) can be projected on a screen. The light rays emanate from the object, go through the lens, and project onto the screen. Real images are always inverted.
A virtual image (right) cannot be projected on a screen. Notice you can see this candle image when looking into the lens itself. Virtual images are always upright.
The thin lens equation relates the focal length, f, to the object distance, s, and the image distance, s'.
A 4.0 cm high object is 150 cm from the nearest focal point a 50 cm lens of a camera as shown.
How far should the film be placed behind the lens to record a focused image?
What is the height of the image on the film?
Draw the ray diagram to show the image.
Note that the image is inverted. What is the magnification?
Notice that your object is more than two focal lengths from the lens.
Which of the following statements is true, concerning a double convex lens?
A. A real image is formed if s < f, a virtual image is formed for s > f
B. A virtual image is formed if s < f, a real image is formed for s > f
C. A real image is always formed
D. A virtual image is always formed
A diverging lens with a focal length of 50 cm is placed 100 cm from an object.
Draw the principle rays
Ray diagrams - summary
The second lens in this optical instrument
A. Causes the light rays to focus closer than they would with the first lens acting alone
B. Causes the light rays to focus farther away than they would with the first lens acting alone
C. Inverts the image but does not change where the light rays focus
Analysis for concave and convex spherical mirrors is similar to that for concave and convex lenses.
Similarly to our lens analysis, for spherical mirrors we draw principle rays from the object to the mirror and then to the image. The diagram above shows the principle rays for a concave mirror. A real, inverted image is formed on the same side of the mirror as the object.
We use the same equations we used for lenses, with a sign convention for mirrors. For a concave mirror, the focal point in front of the mirror is positive. The image distance s' is also positive.
This diagram shows the principle rays for a convex mirror. Notice that here, a virtual, upright image is formed inside the mirror.
The sign convention for a convex mirror is that the focal point beyond the mirror plane is defined to be negative and the image distance is also beyond the mirror plane and is defined to be negative.