Publication No. 12029
In optics, an aperture is a barrier or limitation that reduces the amount of light hitting a lens or mirror and consequently makes the image clearer. Apertures are used in cameras, telescopes and even the human eye. Students will “see” these concepts more clearly as you demonstrate the properties of apertures.
(for each demonstration)
Acetate sheet (optional)
Cards, black, 4½" x 11", 3*
Light source, desk lamp or flashlight
Meter stick (optional)
Mirror support stand*
Tape or poster putty (optional)
*Materials included in kit.
The materials used in this activity are considered safe. Follow all laboratory safety guidelines.
All materials may be saved and stored for future use.
Student Worksheet PDF
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesAsking questions and defining problems
Developing and using models
Planning and carrying out investigations
Engaging in argument from evidence
Disciplinary Core IdeasMS-PS4.C: Information Technologies and Instrumentation
HS-PS4.C: Information Technologies and Instrumentation
Crosscutting ConceptsCause and effect
Scale, proportion, and quantity
Systems and system models
HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
Answers to Questions
An aperture is an opening that restricts the amount of light striking a lens or mirror. This may be done by putting a barrier with a hole in front of the lens. Alternatively, the barrier may be the size of the mirror itself when compared with its focal length. Limiting the amount of light generally increases the clarity of an image. For one, it results in more collimated light. Collimated light essentially means the wave fronts are uniform and even instead of random. Narrowing the opening through which light passes before striking a mirror or lens will result in only the more collimated light making it through (see Figure 3). Additionally, mirrors and lenses all have natural defects known as aberrations. These cause light to reflect more randomly, instead of through the focal point, which interferes with the image. For the same reason as with incoming light, the more scattered reflections or refractions of the outgoing light will likely get filtered out by not making it through the small barrier. Of course, the clarity obtained using an aperture comes at a price—limiting the light and reducing the intensity results in a dimmer image.
Aperture size is calculated using Equation 1.
D is the diameter of the aperture
f is the focal length—in this case, of the concave mirror
f/# is the f-stop number.
Apertures are referred to by their f-stop number, typically in increments of powers of the square root of two—that is:
Each step is referred to as a full stop, and indicates that half of the amount of light is let in. The amount of light allowed through the aperture is proportional to the area. As the area of a circle is πr2, the area of the aperture is given by Equation 2.
It can be seen from this equation that increasing the f-stop number by the square root of 2 will decrease the area by a factor of two, reducing the intensity of the incoming light by one-half.
In photography, different apertures are used for many reasons. In addition to clarifying images, apertures limit the amount of light exposure the film receives, allowing a photographer to use a higher or lower shutter speed with the correct aperture size without overexposing the film. A low shutter speed would require a small aperture, and allows for pictures that can be sharp in both the foreground and the background. A higher shutter speed can have a larger aperture, creating pictures that are sharp only at the right focal length. The aperture size is controlled by an adjustable diaphragm. The aperture in telescopes is not a deliberate barrier, but instead generally refers to the diameter of the main objective mirror (for reflecting telescopes) or lens (for refracting telescopes). For example, the mirror in this demonstration is itself an aperture, as its diameter is smaller than its focal length. With telescopes, larger telescopes generally mean larger apertures. This allows more light to be collected from distant galaxies and stars. A large aperture will not be needed on a telescope for viewing the moon and many of the objects within the solar system, such as the planets, as these reflect a great deal of the light from the sun. But the dim light of distant galaxies and nebula and other deep space objects require as much light as possible to be gathered in order for us to view them. Since the mirror in a telescope must be of such high quality, larger mirrors are far more expensive, which forces a practical balance between aperture diameter and cost. The pupil of the eye is also an aperture. In bright lighting, it narrows to limit the amount of light coming to the eye. In dim lighting, the pupil dilates (increases in size) to allow more light to enter.
Bilash, B. & Maiullo, D. A Demo a Day—A Year of Physics Demonstrations; Flinn Scientific: Batavia, IL, 2009; pp 329–30.