Glossary
Center Ray
A center ray is a light ray that passes directly through the optical center of the lens and continues in a straight line without any deviation.
Example:
The center ray is particularly useful in ray diagrams because it always travels in a straight line, making it easy to draw.
Concave Lens
A concave lens, or diverging lens, causes parallel light rays to spread out after passing through, appearing to originate from a focal point on the same side as the incoming light.
Example:
Eyeglasses for nearsightedness often use a concave lens to spread out light before it reaches the eye, helping distant objects appear clearer.
Convex Lens
A convex lens, also known as a converging lens, causes parallel light rays to bend inward and meet at a focal point on the opposite side of the lens.
Example:
A magnifying glass uses a convex lens to focus sunlight into a tiny, hot spot.
Focal Point
The focal point is a crucial reference point for a lens where parallel light rays converge (for convex lenses) or appear to diverge from (for concave lenses) after refraction.
Example:
For a camera lens, the focal point is where light from a very distant object converges to form a sharp image.
Focal Ray
A focal ray is a light ray that passes through the near-side focal point (for convex) or is directed towards the far-side focal point (for concave) and refracts parallel to the principal axis after exiting the lens.
Example:
For a convex lens, a focal ray starts from the object, goes through the focal point on the same side, and then emerges parallel to the principal axis.
Image Distance ($s_i$)
Image distance ($s_i$) is the distance from the image to the center of the lens, which is positive for real images (formed on the opposite side) and negative for virtual images (formed on the same side).
Example:
A positive image distance indicates that the image formed by the lens can be projected onto a screen.
Image Orientation
Image orientation describes whether the image formed by a lens is upright (having the same orientation as the object) or inverted (flipped relative to the object).
Example:
A negative magnification value always indicates an inverted image orientation.
Image Size
Image size describes whether the image formed by a lens is enlarged (larger than the object), reduced (smaller than the object), or the same size as the object.
Example:
If the absolute value of the magnification is greater than 1, the image size is enlarged.
Image Type
Image type classifies whether the image formed by a lens is real (formed by physically converging light rays) or virtual (formed by light rays that only appear to diverge from a point).
Example:
A projector creates a real image type on a screen, while a magnifying glass creates a virtual one.
Magnification ($M$)
Magnification ($M$) is a dimensionless ratio that compares the height or distance of an image to that of the object, indicating whether the image is enlarged, reduced, upright, or inverted.
Example:
If a lens produces an image that is half the size of the object and inverted, its magnification would be -0.5.
Object Distance ($s_o$)
Object distance ($s_o$) is the distance from the object to the center of the lens, considered positive if the object is on the same side as the incoming light.
Example:
If a student places a toy car 15 cm in front of a lens, the object distance is 15 cm.
Parallel Ray
A parallel ray is a light ray drawn parallel to the principal axis that, after refracting through the lens, passes through the far-side focal point (for convex) or appears to diverge from the near-side focal point (for concave).
Example:
When drawing a ray diagram for a convex lens, the parallel ray is drawn from the top of the object, parallel to the axis, and then through the focal point on the other side.
Ray Diagrams
Ray diagrams are graphical tools that use specific light rays to visually locate and describe the characteristics (orientation, size, and type) of an image formed by a lens.
Example:
Before performing calculations, a physicist might sketch ray diagrams to quickly visualize the expected image location and characteristics.
Real Image
A real image is formed when light rays from an object physically converge at a point after passing through a lens, allowing it to be projected onto a screen.
Example:
The image projected onto a cinema screen by a movie projector is a real image.
Thin-Lens Equation
The thin-lens equation is a formula that relates the object distance ($s_o$), image distance ($s_i$), and focal length ($f$) of a lens, used to calculate any one of these values if the other two are known.
Example:
To determine where a camera's sensor should be placed to get a sharp picture of a distant mountain, a photographer would use the thin-lens equation.
Virtual Image
A virtual image occurs when refracted light rays diverge and only appear to originate from a point, meaning they do not physically converge and cannot be projected onto a screen.
Example:
When you look at your reflection in a flat mirror, you are seeing a virtual image that appears to be behind the mirror.