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In today's lab we will be studying lenses.
We will be working with both converging and diverging lenses.
Converging lenses are thicker in the middle and thinner on the ends.
When parallel light rays are incident on a converging lens these will meet at the focal point.
The focal length is the distance from the lens to this real image
and is positive for converging lenses.
Lenses of different shapes will have different focal lengths.
Stronger lenses have shorter focal lengths.
The power of a lens is measured in diopters, and is equal to 1 over the focal length.
Diverging lens are thinner in the middle and thicker on the ends.
These lenses cause incoming rays to diverge or spread out.
A virtual image forms where their diverse range appear to emerge from.
The focal length for diverging lenses is negative.
One example of a converging lens is found in the eye.
The cornea and lens of the eye, which are both converging,
focus light to form real images on the retina.
Next let's take a look at what happens when we focus an object that is not at infinity.
To do this we'll use a converging lens
and an incandescent light bulb as our object.
To form an image of the object on the screen we can change the object distance, o,
while keeping the image distance, i, constant
until the location of the image matches the location of the screen.
If we look at this setup from another angle, we can clearly see the image go in and out of focus as we move the object.
The distance over which we can move the object while still obtaining a focused image is called the depth of field .
This equation states that one over the focal length is equal to one over the object distance plus one over the image distance.
When using this formula be sure to use the correct sign conventions.
for converging lenses the focal length and object distance are positive
and I can be positive or negative depending on if the image is real or virtual.
For diverging lenses is the focal length is negative
the object distance is positive and the image distance is negative.
We can only form virtual images with diverging lenses.
We can also determine the location of the image graphically by tracing rays
that emerge from a single point on the object
To determine where the image is formed and thus where we should place the screen,
we can draw three easy rays and find where they meet.
We draw the first ray parallel to the optical axis
We can then extend this ray through the focal point.
The second ray travels undeflected through the lens center.
This is because at the center of the lens, the two sides are parallel.
The third ray goes through the focal point on the left side of the lens
and emerges parallel to the optical axis on the right.
An inverted real image forms where these rays converge.
The magnification is the ratio of the image height h i to the object height h o,
and is also equal to the image distance i over the over the object distance o.
In this lab you will study the properties of a variety of lenses
including the focal length, depth of field and chromatic and spherical aberrations.
You will use a light source in the shape of crosshairs as your object.
The ticks on the cross hairs are one millimeter apart.
By using the ruler attached to the optics bench,
you'll be able to measure the image distance and the object distance,
and use these measurements to calculate the focal length.
You will also find the focal length by focusing and object at infinity with the lens
Once you have an image that is in focus, you will measure the size of the image and the size of the object to calculate the magnification.
Additionally you will use this setup to measure the depth of field of the lens
both with and without an aperture placed over the lens.
This aperture wheel allows you to choose between different sizes of aperture.
It can then be snapped onto the lens.
How do you think placing an aperture on the lens will affect the depth of field?