Reflection laws & ray diagrams | Light | Physics class 10 | Khan Academy
Summary
TLDRThis educational video script delves into the laws of reflection, focusing on the second law that equates the angle of incidence to the angle of reflection. It applies these principles to determine the nature and position of images formed by concave and convex mirrors. The discussion includes how specific rays behave when interacting with mirrors, such as those parallel to the principal axis, through the focus, and at the center of curvature. The script concludes with examples of how images appear in different scenarios, including virtual and real images, and their positions relative to the mirror's focal points.
Takeaways
- 🔍 The first law of reflection is that the incident ray, the normal, and the reflected ray all lie in the same plane, but it's not particularly useful for two-dimensional scenarios.
- 📐 The second law of reflection is crucial: the angle of incidence (between the incident ray and the normal) is always equal to the angle of reflection (between the reflected ray and the normal).
- 🌟 If the angle of incidence is 0, the light ray is along the normal, and the angle of reflection will also be 0, causing the light to retrace its path.
- 🔄 For a convex mirror, parallel rays incident on the mirror reflect through the focus, and rays passing through the focus reflect parallel to the principal axis.
- 🎯 A ray through the center of curvature of a spherical mirror travels along the normal and reflects back along the same path, retracing its steps.
- 🖼️ When an object is placed outside the center of curvature of a concave mirror, its image is formed between the center of curvature (C) and the focus (F), and it's real, inverted, and diminished.
- 🤔 If an object is between the pole (P) and focus (F) of a concave mirror, the image is virtual, larger than the object, and cannot be captured on a screen.
- 🪞 For a convex mirror, all reflected rays diverge, and the image formed is virtual, diminished, and erect, appearing inside the mirror but cannot be projected onto a screen.
- 📌 The position and nature of the image depend on the object's placement relative to the mirror's focal points and curvature, with different outcomes for concave and convex mirrors.
- 🧭 Understanding the behavior of light rays as they interact with mirrors is essential for determining the location, size, and orientation of the image formed.
Q & A
What are the two main laws of reflection discussed in the script?
-The first law of reflection is that the incident ray, the normal, and the reflected ray all lie in the same plane. The second law, which is more relevant to the discussion, states that the angle of incidence is always equal to the angle of reflection.
Why is the first law of reflection not useful for the discussion in the script?
-The first law of reflection is not useful for the discussion because it applies to three-dimensional scenarios, and the script focuses on two-dimensional examples.
What does the angle of incidence represent in the context of the script?
-The angle of incidence represents the angle between the incident ray and the normal to the mirror's surface at the point of incidence.
If the angle of incidence is 0, what happens to the reflected ray?
-If the angle of incidence is 0, the ray of light is along the normal, and the angle of reflection will also be zero, causing the ray to go back along the normal, retracing its original path.
How can the position and nature of an image be determined using specific rays of light with a convex mirror?
-By drawing specific rays of light such as those parallel to the principal axis, through the focus, through the center of curvature, and at the pole, one can determine the position and nature of the image formed by a convex mirror.
What is the definition of the focus in the context of mirrors?
-The focus is the point through which all parallel rays of light converge after reflection from a mirror.
What type of image is formed when an object is placed outside the center of curvature of a concave mirror?
-An inverted, real, and diminished image is formed when an object is placed outside the center of curvature of a concave mirror.
What happens to the image when an object is placed between the pole and the focus of a convex mirror?
-When an object is placed between the pole and the focus of a convex mirror, the image formed is virtual, erect, and appears to be larger than the object.
Why are erect images considered virtual?
-Erect images are considered virtual because they cannot be captured on a screen and only appear to diverge from a point behind the mirror.
How does the script explain the behavior of a ray of light aimed at the focus of a convex mirror?
-The script explains that a ray of light aimed at the focus of a convex mirror will, after reflection, appear to go parallel to the principal axis.
What is the significance of the angle of incidence being equal to the angle of reflection?
-The significance of the angle of incidence being equal to the angle of reflection is that it ensures the law of reflection is followed, which is crucial for determining the path of the reflected ray and the formation of images by mirrors.
Outlines
🔍 Laws of Reflection and Image Formation in Mirrors
This paragraph explains the laws of reflection and how they apply to the formation of images in mirrors. The first law, which states that the incident ray, the normal, and the reflected ray all lie in the same plane, is mentioned but deemed not useful for the discussion. The second law, which is crucial, states that the angle of incidence (the angle between the incident ray and the normal) is equal to the angle of reflection (the angle between the reflected ray and the normal). This law is used to predict the size, nature, and position of the image formed by a mirror. The paragraph then demonstrates how to use specific rays of light to determine the image's position for an object placed outside the center of curvature for a convex mirror. Four different rays are considered: one parallel to the principal axis, one passing through the focus, one through the center of curvature, and one directed at the pole. The analysis shows that the image is formed between the center of curvature and the focus, is diminished, inverted, and real, meaning it can be captured on a screen.
🔎 Image Formation with a Concave Mirror
The second paragraph discusses image formation with a concave mirror. It starts by considering the case where an object is placed between the principal axis and the focus (P and F). The paragraph outlines the process of drawing four specific rays of light to determine the image's position: a ray parallel to the principal axis that passes through the focus after reflection, a ray through the focus that becomes parallel after reflection, a ray through the center of curvature that retraces its path, and a ray directed at the pole that reflects with equal angles of incidence and reflection. The analysis reveals that the image is formed inside the mirror, is larger than the object, and is erect, indicating it is a virtual image that cannot be captured on a screen. The paragraph concludes by encouraging the viewer to practice drawing these rays to understand image formation with a concave mirror.
Mindmap
Keywords
💡Laws of Reflection
💡Incident Ray
💡Normal
💡Angle of Incidence
💡Angle of Reflection
💡Convex Mirror
💡Concave Mirror
💡Principal Axis
💡Focus
💡Virtual Image
💡Real Image
Highlights
Exploration of the laws of reflection, focusing on the second law which states that the angle of incidence equals the angle of reflection.
Explanation of how the angle of incidence and reflection affect the size of these angles.
Demonstration that if the angle of incidence is 0, the angle of reflection will also be 0, and the light will retrace its path.
Application of the laws of reflection to convex and concave mirrors to determine image size, nature, and position.
Use of specific rays of light to understand image formation, such as a ray parallel to the principal axis.
Definition of the focus as the point through which all parallel rays of light pass after reflection.
Discussion on how a ray passing through the focus will reflect to become parallel to the principal axis.
Illustration of how rays through the center of curvature reflect straight back, retracing their path.
Explanation that the image formed is between the center of curvature and the focus, and is real and inverted.
Case study of an object placed between the pole and the focus, leading to a virtual image inside the mirror.
Observation that virtual images are erect and larger than the object, and cannot be captured on a screen.
Application of the same ray tracing method to a convex mirror to determine the image's position.
Conclusion that for a convex mirror, parallel rays reflect as if coming from the focus, aiding in image location.
Description of how rays aimed at the focus reflect parallel to the principal axis after reflection.
Final case analysis with an object in front of a convex mirror, resulting in a diminished, erect virtual image.
Transcripts
so let's explore the laws of reflection
the first law says that the incident Ray
which is the ray that strikes the mirror
the normal which is a imaginary
perpendicular that you draw at that
point and the reflected ray all three
Rays lie in the same plane in this
example the plane is that of the
Blackboard now this rule is only useful
if you're dealing in three dimensions
but we're not dealing in three
dimensions so it's not useful for us so
don't worry too much about this law but
the second rule the second law which is
going to be important for us that rule
states that the angle of incidence which
is the angle between the incident Ray
and the normal not this angle but this
angle that will always equal the angle
of reflection the angle between the
reflected ray and the normal and what
this means is that if you have a larger
angle of reflection
angle of incidence you will have a long
larger angle of reflection if you have a
smaller angle of incidence smaller angle
of reflection and if the angle of
incidence is 0 which means the ray of
light is along the normal then the angle
of reflection will also be zero the ray
of light will go back along the normal
retracing its original path now let's
apply these two curve mirrors to find
the size nature and position of the
image so for example imagine we have an
object which is outside the center of
curvature for a convex concave mirror
where will its image be how do we figure
this out or we can draw a few specific
rays of light to help us out the first
ray of light we can draw is parallel to
the principal axis we know that this ray
of light after reflection has to go
through Focus why because that's the
definition of focus it's the point
through which all parallel rays of light
after reflection goes through
so that's the first ray of light I can
draw
then I can draw a ray of light that goes
through the focus if parallel rays of
light goes through the focus after
reflection then a ray of light goes
through the focus after reflection will
go parallel it's just the opposite of
that
and now by looking at these two
reflected Rays you can see that these
two reflected rays meet at this point
and so I know that the tip of the
object's image has to be formed here
and I can draw the image but before I do
that let me draw let me show you a few
other Rays I can draw as well a couple
of more another way I can draw is
through the center of curvature now if a
ray of light goes through the center of
curvature it is going along the normal
remember that normal for a spherical
mirror passes through the center of
curvature so this Ray is along the
normal and we just saw that any ray of
light goes in going along the normal
gets reflected straight back it retraces
the path
and so now this itself becomes the
reflected ray
and finally the fourth Ray that we can
draw is we can shoot array of light
right at the pole and then we know that
the principal axis this time which is
passing through the center of curvature
that acts as the normal and then we can
ensure that the reflected angle has the
same as the incident angle and we can
draw a reflected ray like this
and now you can see all the four
reflected ray pass through the common
point they all meet at one single point
and that is where our image is going to
be
and so we know now that the image is
between C and F it's going to be
diminished inverted and inverted images
are always real images these can be
captured on a screen
so we have solved our problem
okay consider a case where the object is
kept between P and F why don't you pause
and see if you can solve and figure out
where the image is going to be
okay now I'm not gonna draw all four
Rays that's not necessary
okay again one ray of light parallel to
the principal axis goes through the
focus
now the second ray of light where will I
draw I can't draw through the focus
because I know it's not going to hit the
mirror I can't draw through the center
of curvature because that's also not
going to hit the mirror so I'm going to
directly draw the fourth Ray which is
I'm going to shoot it right at the pole
this snap becomes a normal this is the
angle of incidence and then the angle of
reflection has to be equal to the angle
of reflection incidence and then this
becomes the reflected array
and now these are the two reflected
arrays where do they meet notice they
will not meet anywhere which means I
will not get an image on this side
so if I don't see them meeting this side
I will retrace them back and notice they
do meet here which means it appears that
these two rays are coming from here
they're not really but it appears like
that and so now the tip of the objects
uh image is going to be over here
and so this is where the image is going
to be it's inside the mirror
the image is bigger than the object and
you can see the image is erect which
means it's a virtual image virtual
meaning you cannot capture it on a
screen all erect images are virtual
images you can't capture them on a
screen okay one last case you have an
object in front of a convex mirror where
will the image be again try to draw all
the four rays of light that we saw which
we drew for the concave mirror
okay let's do this this might sound a
little tricky but let's do this first
Way of Light I'm gonna pass parallel to
the principal axis where does it go it
doesn't go through the focus because
this is a mirror and it can't go inside
the mirror instead it reflects as if it
appears to come from the focus
can you see that it's very similar to
what we drew earlier parallel trees of
light reflects appearing to come from
the focus
that's the first three
next I'm going to draw array of light
that is aimed at the focus
if a ray of light is aimed at the focus
after reflection it will go parallel to
the principal axis
just like before parallel goes
just like before if the incident Ray is
parallel the reflected ray appears to
come from the focus
and so if the incident Ray is towards
the focus the reflected ray becomes
parallel to the principal axis these two
rays are enough for me to figure out
where the image is going to be but I'm
going to draw a few more Rays the next
thing I'm going to draw is aiming at the
center of curvature
now I know that this is along the normal
which means the reflected ray will
retrace its path it will go along the
same path
and the fourth ray of light I can hit at
the pole making sure that the angle of
incidence and the angle of reflection is
exactly equal and that gives me the
reflected array and again I have these
four rays of light
and where will the image be notice all
the four rays of light appear to be
going away from each other which means
they are not going to meet anywhere so I
have to retrace them back and if I
retrace all of them back look they
appear to be coming from one single
point
so I know that the tip of these objects
here image is going to be somewhere over
here and therefore I'm gonna get the
image here notice it's inside the mirror
between P and F it is diminished smaller
than the object and it is erect which
means it's a virtual image it cannot be
captured on a screen
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