Lens Optics Practice Questions

Lenses and Images

• An image is real if it is on the opposite side of the lens from where light is coming, and virtual if it is on the same side as the light source.

Optical Instruments: The Human Eye

• The human eye consists of a variable-geometry lens (crystalline) that produces a real image on the retina, which is transmitted to the brain via the optical nerve.
• The crystalline lens automatically adjusts itself so we can see objects clearly between infinity and a distance called the "near point" (about 25cm for a typical 20-year-old).
• The "image distance" in the human eye is approximately 2cm, which is the diameter of the eye.

Eyeglasses and Contact Lenses

• A farsighted person needs a convergent lens to correct their vision.
• A nearsighted person needs a divergent lens to correct their vision.
• The "power" of a lens is measured in dioptres: P = 1/f, where f is in meters.

Spherical Lenses

• A lens is an optical device that bends light in a specific way.
• A converging lens bends light so that the light rays come together to a point, forming real and virtual images.
• A diverging lens bends light so it spreads apart, forming only virtual images.

Convex Lenses

• Convex lenses are thicker in the center than at the edges.
• Convex lenses converge light rays, forming real and virtual images.

Concave Lenses

• Concave lenses are thicker at the edges and thinner in the center.
• Concave lenses diverge light rays, forming only virtual images that are erect and reduced.

Ray Tracing for Lenses

• Ray tracing is a method for locating the image formed by a lens.
• The convex lens forms different image types depending on the object's location with respect to the focal point.

Characteristics of Images

• A real image is formed by the actual intersection of light rays from the object.
• Real images can be projected on a surface.
• A virtual image is formed when rays from the same point on an object are bent by the lens so that they appear to come from a much larger object.
• A converging lens acts as a magnifying glass when the object is closer to the lens than one focal length.
• A diverging lens forms a virtual image that is right-side up.
• The magnification varies for single lenses, depending on the distance of the object from the lens.

The Lens Equation

• The lens equation is an alternative to ray tracing for locating the image formed by a lens.