SCIENCE

Quarter 2- Module 5
Light: Lenses
Lesson 1 Images Formed by Lenses
Images Formed ben

What’s New
A lens is a transparent glass with curved sides for concentrating or dispersing light rays.
Some devices which use lenses are cameras, binoculars, telescopes and microscopes. Probably, you
have already tried using a magnifying lens. What have you notice as you vary the distance between
the object and the lens, and the image and the lens? We will be describing the images formed by
lenses based on the location of the object in relation to the lens as we proceed to the lesson.
What is It?
Convex Lens:
A lens that is thicker in the center than at the edges is called a convex lens, they are bent
towards the center.

Image Source:http://www.passmyexams.co.uk/GCSE/physics/concave-lenses-convex-lenses.html

The point at which the light converge is the focal point. Light is refracted as it enters a lens and again
as it leaves the lens. The amount of refraction will depend on the degree to which the lens is curved. A very
curved lens will refract light more than a lens whose surface is only slightly curved. In a very curved lens, the
converging rays will meet at a focal point closer to the lens. Thus, the focal length of a very curved lens is
shorter than that of a slightly curved lens. Because a lens converges light, it can form a real image. However, a
convex lens can also form a virtual image. The kind of image a convex lens forms depends on the position of
the object.
If an object is placed more than one focal length, but less than two focal lengths beyond a convex lens,
the image formed is inverted and larger than the object. An example of this image is the image formed by a
convex lens in a microscope and a slide projector. If an object is placed between a convex lens and it’s focal
point, a virtual image is formed. The virtual image is erect and larger than the object. For this reason, convex
lenses can be used as magnifying glass.
The lens of your eye is a convex lens. It is not a hard and rigid lens, but rather a soft and flexible one.
So, it can easily change shape to allow you to see clear images of objects both near and far.
Ideally, the image formed by the convex lens of the eye should fall directly on the retina. In certain
cases, the image falls in front of the retina because the eyeball is too long. This is called near sightedness. The
lens of a near sighted person is too convex. A concave lens is used to correct near sightedness.
If the eyeball is too short, the image is focused behind the retina. This condition is called
farsightedness. The lens of a farsighted person is not convex enough. The rays of light converge at a point
behind the retina. A convex lens is used to correct farsightedness.
Concave Lens:
A lens that is thicker at the edges and thinner at the center is a concave lens. As parallel rays of light
pass through a concave lens, they are bent toward the edges and the light rays diverge.
All images produced by concave lenses are erect and smaller than the object. Concave lenses are
most often used along with convex lenses to help form a sharper image.
Let us use the lens equation to determine the characteristics of the images formed by lenses. For a
concave lens, the image will always be located on the side where the object is also located. It this case, the
value of image distance is negative. The focal length of a concave lens is negative while positive for a convex
lens. The negative value for image height means an inverted image

When lens equation is used in determining the characteristics of images formed by lenses, we will refer
to this sign convention for lenses. 1
The sign convention for lenses:
• f is + if the lens is double convex
• f is – if the lens is double concave
• q is + if the image is real and located behind the lens
• q is – if the image is virtual and located on the object side of the lens
• h’ is + if the image is upright (also virtual)  h’ is – if the image is inverted (also real)
Where:
• f is the focal length and distance of the object from the lens
• q is the distance of the image from the lens
• h’ is the height of the image
This equation called the mirror equation applies to both concave and convex lenses.
There are two cases which we are going to consider:
1/f = 1/p + 1/q
Where:

f – is the focal length or distance from the mirror and the focal point (F)
p – Distance of the object from the mirror q – distance of the
image from the mirror
Sample Problem:
What is the image distance and image size if a 5.00 cm. tall light bulb placed a distance
of 45.5 cm. from a convex lens having a focal length of 15.4 cm ?
Given Find:

h=5.00 cm. p=45.5 cm. f=15.4

d=? q=?
Substituting the equation: 1/f = 1/p + 1/q
1/15.4 cm=1/45.5 cm.+1/q
1/15.4 cm – 1/45.5 cm. =1/q
45.5 cm-15.4 cm/ (15.4 cm) (45.5 cm.) =1/q

Q= 23.3 cm

Since we now have a value for three of the four quantities in the magnification equation, we can
now calculate the height of the image.

h’/h=-q/p h’/5.00 cm=-23 cm/45.5 cm.

h’= (5.00 cm.) (-23.3 cm.)/45.5 cm h’ =-2.54 cm.

Now for you to have a clearer understanding of the images formed by lenses, if you have
internet connection, watch this video. Watch attentively and take hold of a pen and paper for you to jot
down the characteristics of images formed by lenses. After watching fill up the table below.

Video: Characteristics of Images Formed by Concave and Convex Lens

Link: https://www.youtube.com/watch?v=CJ6aB5ULqa0
If you do not have an internet access to be able to watch the video, examine the figure below to determine the
characteristics of images formed by lenses.

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After watching the video or after examining the ray diagram for lenses, complete the table below.
Table 3: Location, Orientation, Size and Type of Images Formed by Lenses
Location of Object Image

Orientation Size (Same,

Reduced or Type (Real or
Location (Upright or
Enlarged) Virtual)
Inverted)
Convex Lens
Beyond 2F
Between 2F and F
At the Focal Point
Between F and V
Concave Lens:
At 2F
At the Focal Point
Between F and V

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 What’s More
Try solving:
1. Determine the image distance and image height for a 8.00 cm tall object placed at 46.5
cm from a convex lens having a focal length of 16.0 cm.
2. A 3.10 cm. diameter coin is placed a distance of 25.0 cm. from a concave lens that has
a focal length of -11.0 cm. Determine the image distance and the diameter of the
image.
What I Have Learned
1. What happens to the image size and its location as the object comes nearer the concave lens?
convex lens? (5 pts.)
2. A concave lens is also called diverging lens. Why? A convex lens as also called converging
lens. Why? Explain. (5 pts.)
3. How can lenses used to correct the following vision defects? (5 pts.)
a. Farsightedness?
b. Nearsightedness?
What I Can Do
Looking at the data in Table 3, Location, Orientation, Size and Type of Images formed by Lenses,
answer the question below.
Q12. How does the image change in its size and location as the object comes nearer the
convex lens? Concave lens?
Assessment
Choose the letter of the best answer. Write the chosen letter on a separate sheet of paper.
1. The lens which diverges light from a single point.
A. convex lens
B. concave lens
C. biconvex lens
D. all of the above
2. As light passes from air to water, its speed decreases. Describe the refracted ray.
A. It is reflected
B. It travels at the normal line
C. It is bent towards the normal
D. It is bent away from the normal
3. Sun’s rays are observed to focus at a point behind the fishbowl near the window. The fishbowl act
as what type of lens?
A. Converging lens
B. Diverging lens
C. Focusing lens
D. None of the above
4. When light passes from air to glass, which of the following does not change?
A. Frequency B. Speed C. Velocity D. Wavelength
5. Describe the image formed by the human eye.
A. Magnified and real
B. Diminished and real
C. Magnified and virtual
D. Diminished and virtual
6. A person’s legs appear to be shorter when standing waist deep in a swimming pool due to:
A.Diffraction C.Reflection
B. Refraction D.Polarization
7. In most cameras, the location of the image is adjusted for the image to fall on the film by changing
the:
A. Shape of the lens
B. Position of the lens
C. Focal length of the lens
D. Diameter of the diaphragm
8. The human eye is like a camera and hence it contains a system of lens. The eye lens forms
A. an inverted, real image of the object on the retina
B. an inverted, virtual image of the object on the retina

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 C. a straight or upright, real image of the object on the iris
D. a straight or upright, real image of the object on the retina
9. From which part of the lens the ray of light passes without deviation?
A. Pole C. Optical center
B. Focus D. Center of curvature
10. A convex lens can correct:
A. Astigmatism C. Farsightedness
B. Farsightedness D. All of the above
11. The human eye is like a camera and hence it contains a system of lens. The eye lens forms
A. an inverted image infront of the retina
B. an inverted, real image of the object on the retina
C. a straight or upright, real image of the object on the iris
D. a straight or upright, real image of the object on the retina
12. What kind of image is created by a concave lens?
A. upright and smaller C. inverted and larger
B. inverted and smaller D. upright and smaller
13. What kind of lens is used by short-sighted persons?
A. concave B. convex C. compound D. cylindrical
14. What kind of image is formed by a concave lens irrespective of the position of the object?
A. real, upright , diminished C. virtual, upright and magnified
B. real, inverted and magnified D. virtual, upright and diminished
15. Different objects at different distances are seen by the eye. The parameter that remains
constant is
A. the focal length of the eye lens
B. the object distance from the eye lens
C. the radii of curvature of the eye lens
D. the image distance from the eye lens

Answer Key