Many small organisms such as rotifers, copepods and platyhelminths use such organs, but these are too small to produce usable images. Some larger organisms, such as scallops, also use reflector eyes. The scallop Pecten has up to 100 millimetre-scale reflector eyes fringing the edge of its shell.
https://en.wikipedia.org/wiki/Eye
Rather than being an actual part of the eye's anatomy, the pupil is merely an opening. The pupil is the black portion in the middle of the eyeball. Its black appearance is attributed to the fact that the light that the pupil allows to enter the eye is absorbed on the retina (and elsewhere) and does not exit the eye.
http://www.physicsclassroom.com/class/refrn/Lesson-6/The-Anatomy-of-the-Eye
Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eyespot, to allow the organism to deduce the angle of incoming light. Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of simple eye.
https://en.wikipedia.org/wiki/Eye
The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone.
https://en.wikipedia.org/wiki/Eye
In the front of the eyeball is a transparent opening known as the cornea. The cornea is a thin membrane that has an index of refraction of approximately 1.38. The cornea has the dual purpose of protecting the eye and refracting light as it enters the eye.
http://www.physicsclassroom.com/class/refrn/Lesson-6/The-Anatomy-of-the-Eye
The Cornea. The cornea is a transparent dome-shaped tissue that forms the front part of your eye. It functions as a window and allows light to enter your eye. It also begins the process of focusing light rays that allow you to see words and images clearly.
http://www.visionaware.org/info/your-eye-condition/eye-health/anatomy-of-the-eye/125
Light that passes through the pupil opening, will enter the crystalline lens. The crystalline lens is made of layers of a fibrous material that has an index of refraction of roughly 1.40. Unlike the lens on a camera, the lens of the eye is able to change its shape and thus serves to fine-tune the vision process.
http://www.physicsclassroom.com/class/refrn/Lesson-6/The-Anatomy-of-the-Eye
In bright light, the iris closes (or constricts) and makes the pupil opening smaller to restrict the amount of light that enters your eye. The iris in bright light. In dim light, the iris opens (or dilates) and makes the pupil opening larger to increase the amount of light that enters your eye: The iris in dim light. In addition, it is the iris that determines your eye color.
http://www.visionaware.org/info/your-eye-condition/eye-health/anatomy-of-the-eye/125
An alternative to a lens is to line the inside of the eye with mirrors, and reflect the image to focus at a central point. The nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out.
https://en.wikipedia.org/wiki/Eye
You can get a close-up look at different parts of the eye in this eye diagram from the National Institutes of Health. Diagram of the eye, viewed from the side. To understand this diagram of the eye, try to picture it as being split in two, like an apple that's been cut in half.
http://www.visionaware.org/info/your-eye-condition/eye-health/anatomy-of-the-eye/125
what part of the eye allows light to enter
Cornea is a part of the eye that allows light to enter. Cornea is a part of the eye allows light to enter.