Lenses are found in a huge array of optical instruments, ranging from the simple magnifying glass to a camera lens to the lens of the human eye.
The healthy human eye can focus a sharp image of an object on the retina if the object is located anywhere from infinity to a certain point called the near point. The retinal image becomes fuzzy if the object is positioned closer to the eye than this point. The older you are, the further away is this near point, which is approximately 25cm from the eye.
The range of human’s eye can be extended in many ways by optical instruments such as eyeglasses, magnifying glass, telescopes. Instruments like infrared cameras and x-ray microscopes even extends beyond the visible range.
Magnifying Glass: Sunlight focused by a converging magnifying glass can burn the paper. Light rays from the sun are nearly parallel and cross at the focal point of the lens. The more powerful the lens, the closer to the lens the rays will cross.
Large telescopes use reflections to form images of stars and other astronomical objects, whereas Keplerian telescopes (refracting telescopes) use the same principles. The combination of an objective lens (1 in the image) and some type of eyepiece (2 in the image) is used to gather more light than the human eye could collect on its own, focus it 5, and present the viewer with a brighter, clearer, and magnified virtual image 6. The magnification can be found by dividing the focal length of the objective lens by the focal length of the eyepiece.
Practice Questions
Khan Academy
MCAT Official Prep (AAMC)
Physics Online Flashcards Question 5
Key Points
• Optical instruments: including magnifying glass to a camera lens to the lens of the human eye.
Key Terms
Near point: 25cm from the eye. Objects placed closer to the eye from this point will produce fuzzy images on the retina
Keplerian telescope: a refracting telescope usually used in astronomical observations
Magnification: the process of enlarging the apparent size